Ácidos alfa hidroxi

Sinopsis

Alpha Hydroxy Acids (AHAs)

1. Identity: Chemical Names, Natural Sources, and Common Forms

Chemical Definition and Structure

Structurally, AHAs are weak organic acids with one or more hydroxyl groups attached to the alpha carbon, which is the first carbon following the acid group. The name "alpha hydroxy" refers to their shared chemical structure: a hydroxyl group (–OH) attached to the alpha carbon, the carbon atom directly adjacent to the carboxylic acid group. Alpha-hydroxy acids are stronger acids compared to their non-alpha hydroxy counterparts, a property enhanced by internal hydrogen bonding.

Owing to the polar hydroxyl and carboxyl groups in their molecular structure, most AHAs are highly soluble in water, and some are miscible with polar organic solvents such as ethanol and propylene glycol, which this characteristic lays a solid foundation for their dissolution and formulation compatibility in cosmetics.

Principal Members and Natural Sources

Alpha-hydroxy acids (AHAs) are a group of organic acids, originally extracted from fruits, that include compounds such as glycolic acid, lactic acid, citric acid, and lactobionic acid. The family also encompasses malic acid, tartaric acid, and mandelic acid among others used in dermatological and cosmetic practice.

Glycolic acid (alpha-hydroxy acetic acid): The simplest and smallest member of this family, also known by its systematic chemistry name alpha hydroxy acetic acid, is derived from sugarcane and has become the most widely studied AHA in dermatology.
Lactic acid (alpha-hydroxy propionic acid): Lactic acid comes from sour milk or fermented vegetables. Glycolic acid is alpha hydroxy acetic acid; lactic acid is alpha hydroxy propionic acid.
Citric acid: Citric acid is found in citrus fruits such as oranges, lemons, and limes.
Malic acid: Malic acid is derived from fruits like apples, cherries, and apricots.
Tartaric acid: naturally occurring in grapes and certain other fruits, and one of the six main AHAs used in cosmetic formulations.
Mandelic acid: Mandelic acid is one of the largest alpha hydroxy acids, which penetrates the epidermis slowly and uniformly, making it an ideal peeling agent for sensitive skins.

Synthesis and Commercial Production

While a majority of AHAs are obtained from natural sources, gradually, some have also been synthesized. Chemists can also produce alpha hydroxy acids synthetically. For example, glycolic acid and lactic acid are made through chemical synthesis, while citric acid is produced by fermenting a type of mold. One common synthesis hydrolyzes the relatively common alpha-halocarboxylic acids to produce 2-hydroxycarboxylic acids. For instance, glycolic acid is typically produced industrially from chloroacetic acid through base hydrolysis and acid workup. Most cosmetic-grade AHAs are now produced synthetically to ensure consistent purity, concentration, and pH. The natural source tells you where the acid was first isolated, not necessarily where a skincare product's acid comes from.

Molecular Weight and Penetration

The molecular weights and chemical structures of AHAs vary considerably depending on their source. These distinct physicochemical properties resulting from different sources and synthetic methods not only endow various AHAs with unique biological activities but also lay a crucial foundation for their subsequent research and application in the medical field. AHAs are water-soluble, differentiating them from beta hydroxy acids, which dissolve in lipids (fats). This means AHAs primarily work on the skin's top layer to target concerns like wrinkles, discoloration, and dryness.

Common Dosage Forms and Preparations

AHAs are extensively added to various cosmetic products due to their proven efficacy in acne management, with common product categories including skin-renewing toners, serums, lotions, and cleansers. These include daily anti-aging products, such as serums, toners, and creams, as well as occasional concentrated treatments via chemical peels. In clinical settings, these acids are effective anti-aging ingredients, but their use may be limited due to potential side effects like stinging and burning. To address this, second- and third-generation hydroxy acids like gluconolactone and bionic acids have been developed, which provide similar clinical benefits without causing irritation.

2. Traditional and Historical Use

Ancient Practices

The use of exfoliating acids dates back centuries, as far back as ancient Egypt when Cleopatra used to bathe in sour milk to improve her skin's overall look and smoothness. This practice, which harnessed the lactic acid content of fermented dairy, represents arguably the earliest documented use of an AHA for cosmetic benefit, predating modern chemistry by millennia. Old World wine-making cultures similarly applied grape-derived tartaric acid residues and wine to the skin, inadvertently employing tartaric acid, though these practices were not understood in chemical terms until much later.

Modern Scientific Origins (1970s–1980s)

The application of AHAs in dermatology began in 1974, with pioneering research by Dr. Van Scott and Dr. Ruey Yu, who are often referred to as the "fathers of fruit acids." They discovered that AHAs, including glycolic acid, citric acid, and other closely related compounds, had significant dermatological applications. Initial research on alpha-hydroxy acids focused on their use in treating ichthyosis and other dry-skin conditions. Doctors Eugene Van Scott and Ruey Yu introduced the term "alpha-hydroxy acid" and described the effectiveness of AHA products in the topical treatment of ichthyosis and similar dermatoses. Their research reported that alpha-hydroxy acids decreased corneocyte adhesion, allowing removal of the thick scales of ichthyosis, psoriasis, and seborrheic keratoses.

In 1974, Van Scott and Yu indicated that AHAs could have profound effects on disorders related to keratinization. AHAs can be used to easily peel all types of skin with minimal risk. These researchers subsequently described their findings in patents beginning in the late 1970s and early 1980s, describing and claiming the use of alpha hydroxy acids, alpha keto acids, and their derivatives for topical treatment of dry skin. They further described that alpha hydroxy acids and alpha keto acids related to or originating from amino acids were effective in topical treatment of skin disorders associated with disturbed keratinization or inflammation. These skin disorders include dry skin, ichthyosis, palmar and plantar hyperkeratosis, dandruff, Darier's disease, lichen simplex chronicus, keratoses, acne, psoriasis, eczema, pruritus, warts, and herpes.

The carboxylic acids include alpha-hydroxy acids (AHAs), polyhydroxy acids (PHAs), aldobionic acids (ABAs), retinoic acid, vitamin C, and azelaic acid, and they all have therapeutic actions. From the mid-1980s onward, the cosmetic industry increasingly incorporated AHAs into consumer-facing products, transforming them from a dermatological treatment into a widely distributed cosmetic ingredient class. Alpha-hydroxy peels became popular in the dermatologist's arsenal of procedures. Glycolic acid peel is the most common alpha-hydroxy acid peel, also known as fruit peel.

3. Key Constituents and Active Compounds

Classification within the AHA Family

AHAs, PHAs, and ABAs are organic hydroxy acids, a group of natural and physiological substances which can modulate skin keratinization and increase biosynthesis of dermal components. The family is sometimes extended to include polyhydroxy acids (PHAs) such as gluconolactone. A final member of the hydroxy acid family is the polyhydroxy acid, which, as the name suggests, are molecules having at least one carboxylic acid functional group and more than one hydroxyl group. Polyhydroxy acids may be naturally occurring or synthetically manufactured, and have a higher molecular weight than glycolic acid or lactic acid.

Among the conventional AHAs, research on the uses and efficacy of AHAs is extensive. However, out of all the AHAs available, glycolic and lactic acids are the most promising and well researched.

Glycolic Acid

Glycolic acid (MW ~76 Da) is the smallest and most penetrating of the AHAs. Glycolic acid (GA), the smallest and most extensively used of the AHAs, has proven clinically effective at improving the appearance of photodamaged skin, with significant reduction in fine lines and wrinkles, smoothing of rough and uneven skin texture, normalization of skin tone, and reduction in hyperpigmentation.

Lactic Acid

Lactic acid is the second most extensively studied AHA and is notable for its dual moisturizing and exfoliating properties. Studies have confirmed that AHAs exert a pronounced efficacy in alleviating skin xerosis, indicating that AHAs possess moisturizing activity (e.g., lactic acid and lactobionic acid).

Citric, Malic, and Tartaric Acids

The predominant AHAs present in cosmetic products are glycolic acid and lactic acid. Other AHAs used in cosmetic products include citric acid, alpha-hydroxyoctanoic acid, and alpha-hydroxydecanoic acid. Citric acid additionally functions as a pH adjuster in formulations; naturally found in citric fruits and juices, citric acid is used in personal care products to adjust the acidity of the skin, as well as to promote skin turnover and stimulate cell growth.

4. Mechanisms of Action

Corneodesmolysis and Exfoliation

All alpha hydroxy acids work through the same fundamental mechanism: they disrupt corneodesmosomes, the protein structures that hold dead skin cells (corneocytes) together in the stratum corneum, the outermost layer of skin. This process is called corneodesmolysis.

At the enzymatic level, the keratolytic mechanism of AHAs involves the activation of steroid sulfatase and serine proteases, such as kallikrein-related peptidases (e.g., KLK5 and KLK7). These enzymes degrade calcium-dependent adhesion proteins between corneocytes, including desmoglein (DSG), desmocollin (DSC), and corneodesmosin (CDSN). This leads to desmosomal degradation, reduced keratinocyte adhesion, and accelerated skin corneocyte shedding, thereby alleviating follicular keratinization. For instance, glycolic acid can upregulate the KLK7 gene, exerting excellent keratolytic effects.

An alternative and widely cited mechanism involves calcium chelation: the most widely accepted theory is that AHAs remove calcium ions from epidermal cell adhesions by chelation. This results in weakening of the intercellular adhesions, which has an exfoliating effect by causing the shedding and flaking of dead and dry cells. The reduced calcium levels also promote further cell growth while slowing cell differentiation, thereby lessening the appearance of wrinkles and making the skin look younger.

A third mechanism proposed more recently involves ion-channel activation: transient receptor potential vanilloid 3 (TRPV3) channel in keratinocytes is potently activated by intracellular acidification induced by glycolic acid. TRPV3-mediated proton-sensing and cell death in keratinocytes may serve as a molecular basis for the cosmetic use of AHAs.

Concentration-Dependent Keratolysis

Alpha hydroxy acids and alpha keto acids applied topically in lower concentrations reduce the thickness of hyperkeratotic stratum corneum by reducing corneocyte cohesion at lower levels of the stratum corneum. This property permits efficient clinical control of dry skin, ichthyosis, follicular hyperkeratosis, and other conditions characterized by retention of stratum corneum. Applied topically in higher concentrations, these acids cause epidermolysis.

Dermal Remodeling: Collagen, Elastin, and Hyaluronic Acid Synthesis

AHAs can improve wrinkled skin by increasing the synthesis of glycosaminoglycans and thickening skin. At sufficient concentrations, some AHAs — particularly glycolic acid — penetrate deep enough to stimulate biological activity in the living epidermis and dermis. This includes increased keratinocyte proliferation (signaling the skin to produce new cells faster) and fibroblast stimulation, which leads to increased production of collagen and hyaluronic acid. These deeper effects are what give AHAs their anti-aging properties, not just their exfoliating ones.

Research by Okano et al. investigated the effects of glycolic acid on the dermal matrix metabolism of keratinocytes and fibroblasts using in vitro and ex vivo (human skin biopsies) systems. That study showed that glycolic acid not only directly accelerates collagen synthesis by fibroblasts, but that it also modulates matrix degradation and collagen synthesis through keratinocyte-released cytokines. Their experiments confirmed that IL-1a is one of the primary mediators regulating matrix degradation that are released from keratinocytes after glycolic acid treatment.

The hydroxyl groups in AHAs can act as a moisturizing agent, which increases keratinocyte hydration, thereby enhancing the extensibility of the stratum corneum. Furthermore, AHAs can induce the synthesis and release of acid mucopolysaccharides (e.g., hyaluronic acid), thereby enhancing dermal cell hydration and reducing transepidermal water loss (TEWL).

AHAs, such as glycolic and lactic acids, enhance skin rejuvenation by promoting apoptosis in skin cells, boosting collagen and elastin synthesis, and improving skin texture and luminosity.

5. Scientific Evidence by Area of Use

5.1 Photoaging and Skin Aging

Evidence strength: Moderate-to-strong; multiple clinical trials and controlled studies exist, though many are small or of limited duration.

One of the landmark early clinical trials in this area was conducted by Ditre et al. (1996), published in the Journal of the American Academy of Dermatology. The objective was to determine the effects of AHAs on photoaged human skin by clinical and microanalytic means. Patients applied a lotion containing 25% glycolic, lactic, or citric acid to one forearm and a placebo lotion to the opposite forearm for an average of 6 months. Thickness of forearm skin was measured throughout the study, and biopsy specimens from both forearms were processed for analysis at the end of the study. Results showed that treatment with AHAs caused an approximate 25% increase in skin thickness. The epidermis was thicker and papillary dermal changes included increased thickness, increased acid mucopolysaccharides, improved quality of elastic fibers, and increased density of collagen. No inflammation was evident. The conclusion was that treatment with AHAs produced significant reversal of epidermal and dermal markers of photoaging.

In double-blind vehicle-controlled trial settings, AHAs at lower concentrations also demonstrated efficacy. When applied at 8% (glycolic acid or L-lactic acid) for 22 weeks, the majority of patients (76% for glycolic acid; 71% for lactic acid) reported a noticeable improvement in the appearance and smoothness of photoaged skin. Treatment with 12% lactic acid for 3 months has been shown to result in increased epidermal and dermal firmness and thickness, and clinical improvement in skin smoothness and the appearance of lines and wrinkles. No dermal changes were observed after treatment with 5% lactic acid; however, similar clinical and epidermal changes were noted. Further, application of 5% glycolic acid cream for 3 months has been shown to improve skin texture and discoloration of photoaged skin.

A 2021 study published in BMC Dermatology examined glycolic acid adjusted to pH 4, finding it stimulated collagen production and epidermal renewal. The anti-photoaging effects of GA are related to its ability to induce desquamation of the outermost layers of the epidermis. GA reduces cohesion within the stratum corneum by enhancing degradation of the corneodesmosomes responsible for corneocyte adhesion.

An in vitro and ex vivo study confirmed the biochemical basis: the protocol increased epidermal thickness, epidermal and dermal levels of hyaluronic acid, and collagen gene expression. Even small increases in the content of cutaneous hyaluronic acid may result in large changes in epidermal and dermal hydration, affecting skin appearance, texture, and function.

Limitation: The optimal concentrations for therapeutic efficacy with minimal side effects are yet to be precisely defined, necessitating further research.

5.2 Acne Vulgaris

Evidence strength: Moderate; several controlled trials support efficacy, predominantly for glycolic acid peels.

GA may be used in acne also to normalize keratinization and increase epidermal and dermal hyaluronic acid and collagen gene expression. In a study by Wang et al., Asian patients with skin type IV with acne were treated with 35% GA and 50% GA peels, once in 3 weeks for 10 weeks. There was significant resolution of comedones, papules, and pustules. The skin texture improved and follicular pore size reduced.

AHAs, PHAs, and ABAs are organic hydroxy acids, a group of natural and physiological substances which can modulate skin keratinization and increase biosynthesis of dermal components. Because of these effects, AHAs, PHAs, and ABAs are therapeutically effective or beneficial for topical treatment of dry skin, rough skin, acne, rosacea, warts, eczema, psoriasis, and skin changes associated with ageing, including wrinkles and photoageing.

Clinically, as mild keratolytic ingredients, AHAs are less irritating than traditional exfoliants and suitable for most skin types. They can not only act as fundamental functional ingredients in daily skin care but also aid in the improvement of mild-to-moderate acne, thereby enhancing the targeting and effectiveness of skin care regimens.

Limitation: Head-to-head comparisons with other established acne therapies (e.g., benzoyl peroxide, topical retinoids) are limited in number and scope, and most trials are short-term.

5.3 Hyperpigmentation and Melasma

Evidence strength: Moderate; multiple RCTs and comparative trials support glycolic acid peels, often as part of combination regimens.

A multicenter, randomized, double-masked, parallel-group, 24-week clinical study compared the efficacy of the combination of azelaic acid 20% cream and glycolic acid 15% or 20% lotion with hydroquinone 4% in the treatment of facial hyperpigmentation in darker-skinned patients. At week 24, overall improvement and reduction in lesion area, pigmentary intensity, and disease severity were comparable in the two treatment groups. At some visits, patients treated with an azelaic/glycolic acid combination had slightly greater levels of peeling, burning, stinging, or dryness than did patients treated with hydroquinone, although scores for cutaneous signs and symptoms were always low. The study demonstrated that the combination of azelaic acid 20% cream and glycolic acid 15% or 20% lotion was as effective as hydroquinone 4% cream in the treatment of hyperpigmentation in darker-skinned patients.

One controlled study found there was no significant difference between a 35% GA full-face peel performed alone and a 35% GA full-face peel followed by a 10% or 20% TCA spot peel, indicating that combining the peels in the same sitting does not appear to have any synergistic or additive impacts but may raise adverse reactions. Another investigation found that complete regression of diffuse melasma was observed in 30%, partial regression in 60%, and no regression in 10% treated with the combination of 50% GA and 10% kojic acid.

Although their exact mechanism of action is unknown, it has been demonstrated that AHAs improve these disorders by thinning the stratum corneum, promoting epidermolysis, dispersing basal layer melanin, and increasing collagen synthesis within the dermis.

5.4 Dry Skin (Xerosis), Ichthyosis, and Keratosis Pilaris

Evidence strength: Moderate for xerosis; preliminary-to-moderate for keratosis pilaris (KP) and ichthyosis, with limited randomized trial data.

The earliest clinical work on AHAs focused precisely on keratinization disorders. Alpha-hydroxy acids remain an effective treatment for ichthyosis. Formulations containing up to 12% glycolic acid, lactic acid, and its derivatives are typically used. A reduction in epidermal thickness is seen on histological examination within two weeks of the start of twice-daily topical applications of AHAs. A soaking bath once or twice a week helps to remove scales from areas where they are tightly adherent. With sustained treatment, normal or near-normal thickness and appearance of the epidermis can be achieved and maintained.

A controlled clinical study on xerosis, epidermolytic hyperkeratosis, and ichthyosis reported: a study was undertaken to evaluate the safety and efficacy of two novel AHA-containing creams in reducing the appearance and symptoms of problem dry skin on subjects with a range of dry skin conditions, including xerosis, epidermolytic hyperkeratosis, and ichthyosis. Twenty subjects completed a course of treatment with either regular or extra strength AHA-blend cream on a test site, compared with a currently marketed, non-AHA moisturizing lotion on a control site. Subjects were treated for 4 weeks, with clinical evaluations at weeks 0, 2, and 4. The test formulations reduced symptoms and improved cosmetic appearance following 2 weeks of use, with continued improvement following 4 weeks of use.

For keratosis pilaris, topical keratolytics remain reasonable first-line, symptom-directed options for KP, with AHAs, BHAs, and urea all demonstrating potential benefit. However, the overall evidence base is constrained by small sample sizes, heterogeneous outcome measures, limited blinding, and short follow-up, with sparse long-term maintenance data and limited differentiation between texture-dominant and erythematous KP phenotypes.

The theoretical basis for using topical keratolytics (such as AHAs, BHAs, and urea) arises directly from the pathophysiological mechanisms of KP. By promoting exfoliation and improving hydration of the stratum corneum, keratolytics counteract follicular hyperkeratosis and restore a smoother texture. Their ability to soften keratin plugs, enhance desquamation, and improve barrier function aligns closely with the underlying defects in KP, providing a mechanistic justification for their widespread clinical use.

5.5 Skin Moisturization and Barrier Function

Evidence strength: Moderate; mechanistic and clinical data support the role of lactic acid and lactobionic acid in particular.

Studies have confirmed that AHAs exert a pronounced efficacy in alleviating skin xerosis, indicating that AHAs possess moisturizing activity (e.g., lactic acid and lactobionic acid). The moisturizing effect of AHAs is primarily attributed to their hydroxyl groups, which are endowed with robust water-binding capacities. These hydroxyl groups can act as a moisturizing agent, which increases keratinocyte hydration, thereby enhancing the extensibility of the stratum corneum.

AHAs are widely used in cosmetic products to address ultraviolet-related skin damage, having been shown to reduce skin roughness, solar keratosis, and excessive pigmentation, and to increase collagen levels and elastic fibre density. Hydroxy acid-containing products are generally associated with improved exfoliation and increased moisturization.

5.6 Additional Investigated Applications

Because AHAs can modulate skin keratinization and increase biosynthesis of dermal components, they are therapeutically beneficial for topical treatment of conditions including rosacea, warts, eczema, and psoriasis, though the evidence base for these indications is considerably thinner than for photoaging and acne, and largely consists of observational and small clinical data rather than robust randomized controlled trials.

AHAs, such as glycolic and lactic acids, enhance skin rejuvenation by promoting apoptosis in skin cells, boosting collagen and elastin synthesis, and improving skin texture and luminosity. Their comprehensive analysis reveals a nuanced understanding of AHAs' effectiveness across various skin types and conditions, demonstrating their broad utility in treating conditions like acne, hyperpigmentation, and photoaging.

6. Body Systems and Health Areas

AHAs are primarily associated with the integumentary system, with all major documented clinical effects being dermatological. Specific health areas include:

Epidermal barrier and skin hydration: AHAs are natural and physiological substances which can modulate skin keratinization and increase biosynthesis of dermal components.
Dermal matrix and connective tissue: Glycolic acid stimulates fibroblasts to produce collagen and hyaluronic acid, directly affecting the dermis.
Pigmentation system: AHAs disperse basal layer melanin, making them relevant to disorders of melanocyte function including melasma and post-inflammatory hyperpigmentation.
Follicular unit: In the context of KP, AHAs are primarily employed to reduce follicular plugging and improve skin texture rather than to address underlying inflammation. Glycolic acid is capable of correcting abnormal follicular duct keratinization and removing excess keratinocyte buildup, in addition to promoting cutaneous metabolism and facilitating melanin metabolism.
UV and oxidative damage pathways: Whether AHA is a friend or foe of human skin depends on its concentration. These mechanisms of AHAs are currently well understood, aiding the development of novel approaches for the prevention of UV-induced skin damage.

7. Dosage Forms and Concentrations Reported in Studies

Topical Consumer Products

The Cosmetic Ingredient Review (CIR) Expert Panel concluded that products containing the AHAs glycolic and lactic acid are safe for use by consumers if the AHA concentration is 10 percent or less, the final product has a pH of 3.5 or greater, and the final product is formulated in such a way that it protects the skin from increased sun sensitivity or its package directions tell consumers to use daily protection from the sun.

These ingredients are safe for use in salon products at concentrations ≤30%, at final formulation pH ≥3.0, in products designed for brief discontinuous use followed by thorough rinsing from the skin, when applied by trained professionals, and when application is accompanied by directions for the daily use of sun protection.

Concentrations Used in Clinical Studies

A 25% glycolic, lactic, or citric acid lotion applied to one forearm for an average of 6 months in a photoaging trial.
8% glycolic acid or L-lactic acid applied for 22 weeks in a photoaging study; 12% lactic acid applied for 3 months; 5% glycolic acid cream applied for 3 months.
Glycolic acid 15% or 20% lotion combined with azelaic acid 20% cream in a 24-week multicenter RCT for hyperpigmentation.
35% GA and 50% GA peels applied once every 3 weeks for 10 weeks in an acne study.
Formulations containing up to 12% glycolic acid, lactic acid, and its derivatives for ichthyosis.
For treating skin wrinkled and aged by sunlight: alpha hydroxy acid products containing lactic acid, tartaric acid, gluconolactone, or glycolic acid (GA) in 8% concentration are used. The alpha hydroxy acid gluconolactone has also been used in a 14% solution. These products are usually applied to the skin twice daily.

Chemical Peel Concentrations (Professional Use)

FDA analyzed approximately 100 cosmetic products containing AHAs as ingredients and found concentrations of AHAs ranging from 0.01 percent to 67 percent. In professional dermatological peel practice, glycolic acid is commonly used at concentrations ranging from 20% to 70%, as reflected in multiple published clinical trials. Alpha-hydroxy acid peels can be neutralized with water or with basic solutions, such as ammonium salts, sodium bicarbonate, or sodium hydroxide.

8. Safety Considerations and Notable Interactions

Photosensitivity

The most thoroughly documented safety concern with topical AHAs is increased sensitivity to ultraviolet (UV) radiation. Reviewers evaluated human clinical studies that investigated the effects of ultraviolet (UV) radiation on the skin after exposure to AHAs. The studies demonstrated that topically applied AHAs increase skin sensitivity to UV radiation during application and that this increased skin sensitivity to UV radiation diminishes after discontinuing application for a week.

In 2005, the FDA published a guidance document which specifies that due to the potential for photosensitivity, products containing AHAs should bear a Sunburn Alert on product packaging. The required label language states that the product contains an AHA that may increase skin sensitivity to the sun and particularly the possibility of sunburn. Use of a sunscreen, wearing protective clothing, and limiting sun exposure while using such a product and for a week afterward is recommended.

Regarding carcinogenic risk, a study by the FDA and the National Toxicology Program determined that glycolic acid did not affect photocarcinogenesis (the development of cancer cells associated with exposure to light) in mice.

Irritation, Burning, and Adverse Reactions

Caution should be exercised in relation to certain adverse reactions among patients using products with AHAs, including swelling, burning, and pruritus. The use of AHAs in cosmetic products can lead to certain adverse reactions, including swelling, burning, and pruritus. High concentrations of AHAs can cause irritation, with studies suggesting that concentrations above 10% can lead to increased risk of adverse effects.

One product for which FDA received five adverse experience reports (e.g., skin irritation, burning) contained only 0.3% alpha-hydroxydecanoic acid and 0.4% alpha-hydroxyoctanoic acid, for a total of 0.7% AHAs, suggesting that AHAs may be associated with adverse reactions even at these low concentrations.

Side effects including itching and burning sensations are particularly noted in the management of congenital ichthyoses, where AHAs are used as keratolytics. The use of AHAs, typically in concentrations of 5–12%, can lead to these adverse effects, which may necessitate tapering the application or discontinuing use. Application on the face, flexures, and areas of fissuring is not recommended due to the risk of irritation.

Role of pH and Formulation

The acidity of the formulation is a critical safety variable. Compositions containing these acids may irritate human skin on repeated topical applications due to the lower pH of the formulations. The CIR safety thresholds therefore specify not only maximum concentration but also minimum pH: in 1998, the Panel concluded that glycolic and lactic acid, their common salts and their simple esters, are safe for use in cosmetic products at concentrations ≤10%, at final formulation pH ≥3.5, when formulated to avoid increasing sun sensitivity or when directions for use include the daily use of sun protection.

Long-Term Safety and Barrier Integrity

The safety of long-term use has not been established. There are limited studies in maintenance of barrier integrity and limited information on effects on absorption of other cosmetic ingredients. The European Commission's Scientific Committee on Cosmetic Products and Non-Food Products (SCCNFP) identified these knowledge gaps in their review and noted that further research was warranted.

Interactions with Other Topical Agents

In patients with photodamage, AHA peels and topical products are often combined with retinoids and other antioxidants for maximum benefit. Combining AHAs with retinol could offer a promising anti-aging solution, particularly for acne-prone skin. However, the combination of AHAs with retinoids may heighten the risk of irritation and barrier disruption, as both classes individually increase skin turnover and sensitivity. Whether AHA is a friend or foe of human skin depends on its concentration, a principle that also applies to combination regimens: the overall acid burden and pH of all co-applied products is clinically relevant.

The formulation of AHA-based cosmetic products requires careful consideration of factors such as molecular size, solubility, pH, and concentration.

Skin Type Considerations

Sensitive skin: Alpha hydroxy acids can worsen skin conditions by causing skin irritation and removal of the top layer of skin cells. Individuals with darker skin phototypes may also be at increased risk of post-inflammatory hyperpigmentation following higher-concentration AHA peels, as reflected in clinical protocols recommending pre-peel skin preparation and careful patient selection. Careful review of medical history, examination of the skin, and pre-peel priming of skin are important before every peel.