Furazadrol

Furazadrol: A Comprehensive Reference Article

1. Identity and Chemical Characterization

1.1 Names and Nomenclature

Furazadrol is a synthetic steroidal compound that circulates under several chemical names in the scientific and commercial literature. Its principal systematic IUPAC-derived designation is [1′,2′]isoxazolo[4′,5′:2,3]-5α-androstan-17β-ol, and it is formally classified as a designer anabolic androgenic steroid. The NIH Office of Dietary Supplements Dietary Supplement Label Database (DSLD) records the following terms as labels applied to this ingredient on commercial supplement products: 5a-androstanol[2,3]furazan-17b-tetrahydropyranol; 5a-androstanol[2,3]furazon-17b-tetrahydropyranol; 5a-etioallocholan 2,3-c furazan-17b-tetrahydropyranol; 5a-etioallocholan(2,3-c)furazan 17b-tetrahydropyranol ether; Furazadrol; Orastan-A.

The NIH's Global Substance Registration System (GSRS/NCATS) records furazadrol with the molecular formula C₂₀H₂₉NO₂ and a molecular weight of 315.45 g/mol, with seven defined stereocenters and no charge. Its IUPAC name as registered is 1H-Cyclopenta[7,8]phenanthro[3,2-d]isoxazol-1-ol, 2,3,3a,3b,4,5,5a,6,10,10a,10b,11,12,12a-tetradecahydro-10a,12a-dimethyl-. A second chemical entity marketed as "Furazadrol" — specifically the tetrahydropyranyl (THP) ether prodrug form — carries a different molecular formula; chemical reference suppliers record it as C₂₄H₃₆N₂O₃ with a molecular weight of 400.55 g/mol and CAS number 4975-12-6, its IUPAC name being (1S,3aS,3bR,5aS,10aS,10bS,12aS)-10a,12a-dimethyl-1-((tetrahydro-2H-pyran-2-yl)oxy)-2,3,3a,3b,4,5,5a,6,10,10a,10b,11,12,12a-tetradecahydro-1H-cyclopenta[7,8]phenanthro[2,3-c][1,2,5]oxadiazole.

Furazadrol is a derivative of dihydrotestosterone (DHT) containing an isoxazole ring fused to the steroid A-ring. It is important to note that the "furazan" ring described in early commercial labeling and a true "isoxazole" ring are distinct structural features; scientific analysis has consistently identified the compounds in commercial products as the isoxazole isomers rather than the furazan structure claimed on labels (see Section 4 below).

Two principal isomeric forms are recognized in the peer-reviewed literature: steroid isoxazoles such as [1′,2′]isoxazolo[4′,5′:2,3]-5α-androstan-17β-ol (referred to as furazadrol F) and its isomer [1′,2′]isoxazolo[4′,3′:2,3]-5α-androstan-17β-ol (isofurazadrol IF) have been detected as components of so-called "dietary supplements."

1.2 Natural Source

Furazadrol does not have a long-standing tradition in ancient medicinal systems or folk remedies. It is a wholly synthetic compound with no botanical or natural origin. The NIH DSLD classifies it in the category "Non-nutrient/non-botanical." Its category on the NIH Dietary Supplement Label Database is listed as "Non-nutrient/non-botanical," with no scientific resources cited.

1.3 Commercial Forms and Preparations

Furazadrol has been reported as a component of dietary supplements such as Orastan-A (Gaspari Nutrition) and Furazadrol (Axis Labs), predominantly as the tetrahydropyranyl ether. In both cases, these supplements had incorrect labelling of the content information. The THP ether is a prodrug form; following oral administration, these THP ethers are expected to easily undergo hydrolysis by gastric acid. Commercial preparations have generally been sold as oral capsules.

During the early-to-mid 2000s, more and more products appeared on the market containing steroids that were never approved as therapeutic drugs, mostly without proper labeling of the contents. Furazadrol was introduced as a "prohormone" alternative, intended to provide anabolic effects similar to those of anabolic steroids but with a different structure to bypass certain regulatory restrictions, and its popularity grew in the mid-2000s among athletes and fitness enthusiasts seeking enhanced performance and physique improvements.

2. Historical and Traditional Context

2.1 Synthetic Origins

Furazadrol is an entirely synthetic molecule with no history of traditional or ethnobotanical use. Early literature references to isoxazole-fused androstane structures date to Japanese pharmaceutical chemistry publications from the 1960s. Chemical reference suppliers cite foundational syntheses in Chemical and Pharmaceutical Bulletin: Kasahara, A., et al., Chem Pharm Bull 16, 1456 (1968) and Ohta, G., et al., Chem Pharm Bull 13, 1445 (1965). These were pharmacological research compounds, not drugs developed for clinical therapeutic use, and they were never brought to market as approved pharmaceuticals.

2.2 Modern Use in Sports Nutrition

Furazadrol is a synthetic compound that has been incorporated into various nutritional products, particularly within the fitness and bodybuilding communities. Historically, Furazadrol emerged as a prohormone designed to mimic certain anabolic effects while offering a legal alternative to more strictly regulated substances. Furazadrol was developed by Axis Labs as an alternative to methylated prohormones. As a non-methylated compound it has appealed on grounds of being marketed as very safe and is often used by beginners new to prohormones, either alone or in a stack with other compounds.

The DSLD of the NIH Office of Dietary Supplements documents its presence on the labels of multiple U.S. dietary supplements, confirming its documented circulation within regulated markets under the U.S. Dietary Supplement Health and Education Act (DSHEA) framework, even as the scientific content of these products was found to differ from their labels.

3. Key Constituents and Structural Features

3.1 The Isoxazole A-Ring Fusion

The defining structural feature of furazadrol is the fusion of an isoxazole heterocycle onto the A-ring of the androstane (DHT-derived) steroid nucleus. This isoxazole fused to the steroid A-ring offers metabolic stability and noteworthy anabolic activity, raising concerns over the potential for abuse of this compound in equine sports. The structural analogy to the well-known pharmaceutical danazol (17α-ethinyl-17β-hydroxyandrost-4-eno[2,3-d]isoxazole) is recognized in the scientific literature, though furazadrol and danazol differ at the C-17 position and ring system.

3.2 Isomeric Composition in Supplements

Scientific analysis of commercial products claiming to contain furazadrol has revealed that the actual steroidal content differs systematically from label claims. The product Orastan-A from Gaspari Nutrition was analyzed for its steroid content. According to the labeling, it is supposed to contain "5a-Androstano[2,3-c]furazan-17b-tetrahydropyranol ether," also called furazadrol-THP ether. GC-MS analyses of the liberated steroids (after extraction from the capsule matrix and cleavage of the THP ether, TMS-derivative and underivatized) revealed mass spectra of two components, both inconsistent with the labeling. The steroids were characterized by different analytical techniques such as mass spectrometry, nuclear magnetic resonance spectroscopy and X-ray crystal structure analysis, and were identified as 17β-hydroxyandrostano[3,2-c]isoxazole and -[2,3-d]isoxazole.

The synthesis of these isomers is described in peer-reviewed methods. Isoxazole formation in ethanol solvent affords a 10:1 mixture of furazadrol F and isofurazadrol IF that is not readily separable by column chromatography or purified by recrystallization. Given that materials sourced online are reported to contain both isomeric steroids in varying ratios, this mixture was deemed suitable to conduct in vivo and in vitro metabolism studies.

4. Mechanisms of Action

4.1 Androgen Receptor Activation

Both F (furazadrol) and IF (isofurazadrol) have been reported to exert anabolic activity in the older steroid literature, in a mode-of-administration-dependent manner, and more recently in both yeast and human HuH7 androgen bioassays, with furazadrol F reported to display greater activity than its isomer.

Quantitative data on relative androgenic potency was established in a 2011 study published in Analytical Chemistry (Waller et al.), which employed stably transfected HEK293 and HuH7 cell lines. Two new in vitro androgen bioassays were developed by stably transfecting HEK293 and HuH7 cells with the human androgen receptor (hAR) expression plasmid together with a novel reporter gene vector (enhancer/ARE/SEAP). The yeast β-galactosidase androgen bioassay was used for comparison. The relative potencies (RP), defined as [EC50] of testosterone/[EC50] of steroid, of the Furazadrol nutraceutical extract in the yeast, HEK293-S, and HuH7-S assays were 208, 250, and 80, respectively. These RP values indicate that furazadrol was 208-fold less potent than testosterone in the yeast system, 250-fold less potent in HEK293 cells, and 80-fold less potent in HuH7 liver cells — meaning that higher concentrations of furazadrol were required to achieve the same receptor activation as testosterone in these in vitro systems.

The isomeric composition of furazadrol supplements affects bioactivity. Both isomers (the [3,2-c]isoxazole and the [2,3-d]isoxazole) are strong androgens in the yeast bioassay; however in the HuH7 bioassay, the [3,2-c]-isomer was 3-fold more potent than the [2,3-d]-isomer, with metabolic activation of the former but a trend towards deactivation for the latter.

4.2 Structural Basis for Metabolic Stability

Furazadrol has a unique structure, made up of an isoxazole ring fused to the A-ring of a steroid, enabling metabolic stability and significant anabolic activity. The isoxazole fusion replaces the conventional 3-keto-Δ4 functionality present in testosterone and many classical steroids, which is a primary site for metabolic deactivation. This structural modification is understood to confer resistance to first-pass hepatic metabolism compared with certain classical steroid hormones.

4.3 Non-Aromatization

Unlike some other anabolic agents, furazadrol is thought to have minimal to no conversion to estrogen, which may reduce certain side effects. This property is a chemical consequence of the lack of the 4,5-unsaturated 3-ketone system required for aromatase-mediated conversion to estrogens, since furazadrol's A-ring is substituted with the isoxazole moiety.

5. Scientific Evidence by Area of Application

5.1 Anabolic / Muscle-Building Activity

Evidence type: In vitro bioassay data only; no human or animal clinical trials.

From a scientific perspective, furazadrol's anabolic potential is believed to stem from its structural similarity to potent anabolic steroids, suggesting it may contribute to increases in lean muscle mass and strength. However, direct clinical studies on furazadrol in humans are lacking, and most evidence is anecdotal or extrapolated from its chemical relatives.

The in vitro bioassay data described above (Section 4.1) provide the most rigorous available evidence for anabolic androgenic receptor activity. Most sport supplements containing furazadrol are marketed online and may contain undisclosed steroids yet are readily available despite lacking toxicological or pharmacological evaluation. In studies examining 18 supplements purchased online, organic solvent extraction was used to isolate steroids, and for each, its intrinsic androgenic potency was determined by a yeast cell (Saccharomyces cerevisiae) androgen bioassay and its potential androgenic potency was determined by a liver (HuH7) cell androgen bioassay. The yeast bioassay showed that of the 19 steroids tested across 18 supplements, 6 demonstrated strong intrinsic bioactivity, with 4 metabolically activated to even stronger androgens. Moreover, 4 steroids with moderate and 1 with intrinsically weak androgenic bioactivity were activated to more potent androgens.

Despite these potential advantages, the scientific validation of furazadrol's efficacy and safety remains limited. The absence of robust clinical trials means that its long-term effects and overall health impact have not been fully elucidated.

Evidence strength assessment: Weak. Evidence is limited to in vitro cell-based androgen receptor bioassays. No controlled human clinical trials, randomized controlled trials, or animal feeding studies demonstrating changes in lean body mass, strength, or body composition attributable to furazadrol have been identified in the peer-reviewed literature as of the available searches.

5.2 Performance Enhancement in Animal Sports

Evidence type: In vivo animal metabolism studies (racehorses, greyhounds); controlled oral administrations.

The majority of peer-reviewed research on furazadrol has been conducted in the context of equine and canine anti-doping science. The metabolism of furazadrol was studied by in vivo and in vitro methods for the first time, with urinary furazadrol 17-sulfate and furazadrol 17-glucuronide metabolites detected in vivo after a controlled administration, compared with synthetically-derived reference materials in order to confirm their identities.

Following an equine in vivo controlled oral administration of 200 mg in one thoroughbred gelding, furazadrol was excreted primarily as the sulfate and glucuronide conjugates without phase I metabolism, which were detectable up to 24 hours post-administration by LC-HRAM analysis.

A 2021 study published in Drug Testing and Analysis (Pranata et al., PMID 34254454) extended this work to the greyhound model. The metabolism of furazadrol (F:IF ratio 10:1) was studied by in vivo methods in greyhounds, with urinary phase II furazadrol metabolites detected as glucuronides after a controlled administration. Using a library of synthetically-derived reference materials, the identities of seven urinary furazadrol metabolites were confirmed. Major confirmed metabolites were isofurazadrol IF, 4α-hydroxyfurazadrol (4α-HF), and 16α-hydroxy oxidised furazadrol (16α-HOF), while the minor confirmed metabolites were furazadrol F, 4β-hydroxyfurazadrol (4β-HF), 16β-hydroxyfurazadrol (16β-HF), and 16β-hydroxy oxidised furazadrol (16β-HOF). One major hydroxyfurazadrol and two dihydroxyfurazadrol metabolites remained unidentified.

Evidence strength assessment: The in vivo animal metabolism studies are scientifically rigorous with respect to pharmacokinetics and detection, but they were conducted for anti-doping purposes, not to evaluate therapeutic efficacy. No human pharmacokinetic or pharmacodynamic studies have been published.

6. Body Systems and Health Areas of Association

6.1 Musculoskeletal System

Furazadrol is marketed primarily for its purported effects on skeletal muscle: specifically, increases in lean mass, muscle hardness, and strength. In the context of wellness and fitness, furazadrol has been utilized for its purported ability to enhance muscle hardness, strength, and lean mass retention. These claims rest exclusively on the compound's androgen receptor activity demonstrated in vitro and by structural analogy with better-studied androgens; no controlled intervention studies in humans establishing these outcomes have been published in peer-reviewed literature.

6.2 Endocrine System

As an androgen receptor agonist, furazadrol would be expected to interact with the hypothalamic-pituitary-gonadal (HPG) axis, potentially suppressing endogenous testosterone production through negative feedback — a well-established effect of exogenous androgens and anabolic steroids as a class. Supraphysiologic and long-term use of anabolic-androgenic steroids affects all organs, leading to cardiovascular, neurological, endocrine, gastrointestinal, renal, and hematologic disorders. No furazadrol-specific human data on HPG axis suppression exist in the published literature.

6.3 Hepatic System

Hepatotoxicity is one of the major concerns regarding anabolic-androgenic steroid treatment and abuse. Testosterone and its derivatives have been most often shown to induce a specific form of cholestasis, peliosis hepatis, and hepatic benign and malignant tumors. It is currently believed that mechanisms of pathogenesis of these disorders include disturbance of antioxidative factors, upregulation of bile acid synthesis, and induction of hepatocyte hyperplasia. Most toxicity cases are treated with supportive measures and liver function normalizes with discontinuation of AAS. No furazadrol-specific hepatotoxicity data in humans have been published in the peer-reviewed literature; however, the compound's classification as an AAS-type molecule places it within a class for which hepatotoxicity is a recognized concern.

6.4 Cardiovascular System

Androgen-mediated lipid alterations — particularly reductions in high-density lipoprotein (HDL) cholesterol — are a well-documented adverse effect of anabolic-androgenic steroids. Case reports of service members using prohormone supplements include cases with asymptomatic hepatocellular toxicity and marked dyslipidemia; aspartate aminotransferase (AST) and alanine aminotransferase (ALT) normalized 4 weeks after supplement cessation, with HDL and LDL returning to baseline at 8 weeks. These are class effects reported in the context of prohormone/AAS use generally and are not specific to furazadrol alone.

7. Regulatory and Anti-Doping Status

7.1 WADA Prohibited List

Related isoxazole-containing steroids, including the structurally similar danazol, are banned in competition by the World Anti-Doping Agency (WADA) and the International Federation of Horseracing Authorities. The WADA Prohibited List specifies that any pharmacological substance not addressed by other sections and with no current approval by any governmental regulatory health authority for human therapeutic use — including designer drugs — is prohibited at all times. Furazadrol, as a designer anabolic androgenic steroid without approved therapeutic use, would fall under this category in addition to the general S1 Anabolic Agents prohibition.

7.2 Supplement Mislabeling

Structural changes to the core steroid structure help suppliers evade legal restrictions and penalties regarding their manufacture and sale in some jurisdictions. The mislabeling discovered in scientific analyses of commercial products has anti-doping and public health implications. Products containing unapproved steroids that had already been synthesized in the 1960s appear on the market of sport supplements, often labeled as "dietary" substances.

Supplement adulteration with anabolic-androgenic steroids has been reported and AAS-associated drug-induced liver injury is clinically variable. The NIH DSLD categorizes furazadrol as a non-botanical, non-nutrient ingredient, with no scientific resources cited — reflecting the absence of government or institutional endorsement of any health claims.

8. Pharmacokinetics and Metabolism

8.1 Phase I Metabolism

In the equine model, minor metabolites detected included epifurazadrol, hydroxylated furazadrol, and hydroxylated and oxidised furazadrol, present as the sulfate and glucuronide conjugates. The in vitro phase I metabolism profile closely mirrored in vivo findings: the metabolism profile was compared to the products obtained from an in vitro phase I metabolism study, with all but two of the minor in vivo phase I metabolites observed in the in vitro system.

8.2 Phase II Metabolism and Urinary Excretion

Urinary furazadrol 17-sulfate and furazadrol 17-glucuronide metabolites were detected in vivo after a controlled administration and compared with synthetically-derived reference materials in order to confirm their identities. They were quantified to establish the excretion profile and a suitable limit of detection. In the thoroughbred racehorse model, furazadrol 17-sulfate (FS) and furazadrol 17-glucuronide (FG) metabolites were detected in vivo up to one day following controlled oral administration.

In the greyhound model, the metabolite profile was more extensive. Using a library of synthetically-derived reference materials, the identities of seven urinary furazadrol metabolites were confirmed. Major confirmed metabolites were isofurazadrol IF, 4α-hydroxyfurazadrol 4α-HF, and 16α-hydroxy oxidised furazadrol 16α-HOF, while the minor confirmed metabolites were furazadrol F, 4β-hydroxyfurazadrol 4β-HF, 16β-hydroxyfurazadrol 16β-HF, and 16β-hydroxy oxidised furazadrol 16β-HOF. One major hydroxyfurazadrol and two dihydroxyfurazadrol metabolites remained unidentified. Qualitative excretion profiles, limits of detection, and extraction recoveries were established for furazadrol F and major confirmed metabolites.

Phase II metabolites were subjected to enzymatic hydrolysis by Escherichia coli β-glucuronidase and Pseudomonas aeruginosa arylsulfatase to further confirm the identity of the corresponding phase I metabolites.

9. Dosage Forms and Reported Dosages

Furazadrol has been sold commercially in oral capsule form, predominantly as the tetrahydropyranyl (THP) ether prodrug. In terms of dosage, no human clinical studies establishing safe or effective doses exist in the peer-reviewed literature. The only controlled animal administration dose identified in the published scientific literature is the equine study: an equine in vivo controlled oral administration of 200 mg in one thoroughbred gelding.

The NIH DSLD lists furazadrol as an ingredient without specifying a standard serving dose, as the database records label information rather than clinically validated doses. Any dosage figures appearing on commercial supplement labels or non-peer-reviewed sources cannot be verified against clinical evidence and are therefore outside the scope of this reference.

10. Safety Considerations and Interactions

10.1 Absence of Approved Safety Data

Furazadrol has never received approval from any governmental regulatory health authority — including the U.S. FDA, the European Medicines Agency, or the WHO — for human therapeutic use. In recent years the potential for anabolic steroid abuse in equine sports has increased due to the growing availability of designer steroids. These compounds are readily accessible online in "dietary" or "nutritional" supplements and contain steroidal compounds which have never been tested or approved as veterinary agents.

10.2 Androgenic Side Effects

Furazadrol is a DHT-derived androgen receptor agonist. As a consequence, androgenic adverse effects associated with DHT — including promotion of androgenetic alopecia and potential effects on the prostate — are mechanistically plausible. Unlike some other anabolic agents, furazadrol is thought to have minimal to no conversion to estrogen, which may reduce certain estrogenic side effects. However, the absence of estrogenic conversion does not preclude androgenic adverse effects.

10.3 Hepatotoxicity Risk

Although furazadrol is marketed as "non-methylated" — a distinction sometimes used to imply reduced liver strain compared with 17α-alkylated steroids — this does not eliminate hepatotoxicity risk. Since the 1950s, a significant number of studies and case reports have raised concerns regarding hepatotoxic effects including cholestasis, hepatic neoplasms, hepatocyte toxicity, and peliosis hepatis related to use of anabolic-androgenic steroids. Correlation between steroids and these effects has been attributed to the role of the liver as the primary clearance site for AAS. Synthetic steroids are usually modified to prevent first-pass hepatic metabolism, preventing their clearance by the liver and, therefore, elevating their risk of hepatotoxicity.

10.4 Supplement Adulteration and Mislabeling

A specific and documented safety concern with furazadrol products is product mislabeling. Scientific analyses have consistently found that the actual steroidal content of commercial products differs from label claims. Supplement adulteration with anabolic-androgenic steroids has been reported and AAS-associated drug-induced liver injury is clinically variable. Cases of AAS-associated drug-induced liver injury in deployed service members have been documented, highlighting variable hepatotoxicity patterns of AAS, concern with inaccurate supplement labeling, and the need for educational resources.

10.5 Anti-Doping Violations

Samples of the dietary supplement "Furazadrol" sourced through the internet have been reported to contain the designer anabolic androgenic steroids furazadrol F and isofurazadrol IF. These steroids contain an isoxazole fused to the A-ring and were designed to offer anabolic activity while evading detection, raising concerns over the potential for abuse of this preparation in sports. Designer steroids such as furazadrol pose a significant threat to the integrity of sport if left unchecked. Athletes subject to anti-doping testing who use products labeled as furazadrol face a material risk of an adverse analytical finding, given that furazadrol and its metabolites are detectable in urine and that the structurally related class of exogenous anabolic androgenic steroids is prohibited by WADA.

11. Summary of Evidence Profile

Furazadrol is a wholly synthetic, non-botanical designer androgenic anabolic steroid with no approved clinical use in humans or animals. Sport supplements containing steroids never approved for therapeutic use have the potential for abuse by athletes. The peer-reviewed evidence base for furazadrol consists exclusively of:

• Chemical characterization and analytical identification studies (GC-MS, NMR, X-ray crystallography) of commercial supplement products.
• In vitro androgen receptor bioassay data demonstrating androgenic activity in yeast, HEK293, and HuH7 cell lines.
• In vivo pharmacokinetic/metabolism studies in thoroughbred racehorses and greyhounds, conducted for anti-doping purposes.

No peer-reviewed randomized controlled trials, controlled human intervention studies, or systematic reviews of human clinical data exist for furazadrol. All claims regarding benefits for muscle gain, body composition, or athletic performance in humans are unsubstantiated by controlled clinical evidence.

References

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