Purple milkweed

Purple Milkweed (Asclepias purpurascens L.): A Comprehensive Reference

1. Identity and Botanical Description

Accepted Scientific Name and Taxonomy

Asclepias purpurascens L. is the accepted scientific name for purple milkweed, with the genus and species name first described by Carl von Linnaeus in the first volume of the first edition of Species Plantarum in 1753. It is a member of the plant family Apocynaceae. The species was formerly placed in the family Asclepiadaceae, a classification still encountered in older botanical literature. Synonyms recorded in Plants of the World Online include Asclepias amoena L. (1753), Asclepias compressa Moench (1794), and Asclepias dasypus Raf. (1832), among others.

The genus Asclepias is named after the Greek god of healing, Asklepios, the originator of the staff and serpent commonly associated with medicine today. The specific epithet purpurascens means "purple."

Common Names and Distinguishing Morphology

The plant is known by the common name purple milkweed. Asclepias purpurascens is a herbaceous plant species and is a type of milkweed in the genus Asclepias. The plant gets its name from the flowers that first develop a pink color but then turn darker purple as they mature. The plant grows 2 to 3 feet tall and wide in full sun to partial shade.

Purple milkweed is a perennial forb of woodlands, savannas, prairies, and wetlands; leaves are opposite on short petioles with dense short hairs below, tips acute; flowers are purple, borne in 1–3 terminal clusters. The oppositely arranged leaves are pubescent and dark green on the upper surface and densely pubescent and light green on the lower surface. Like other milkweeds, this plant produces a bitter-tasting and potentially irritating white latex. The species rarely produces seed pods, which are smooth, instead of the rough warty ones produced by common milkweed.

Distribution and Natural Range

Asclepias purpurascens is native to the Eastern, Southern and Midwestern United States, similar to the range of the common milkweed (Asclepias syriaca). A member of the Apocynaceae family, purple milkweed is found growing primarily in Eastern North America. Preferred habitat includes full to part sun on sandy soils of prairies, shrub thickets, shores, and dry open woodlands such as oak and oak/pine forests. Unlike common milkweed, purple milkweed prefers some shade and is considered a plant of partial shade. It is also considered an indicator of oak savanna, especially in Wisconsin.

Conservation Status

Asclepias purpurascens is listed as endangered in Massachusetts and Wisconsin, officially as historical to Rhode Island (though with two recently discovered yet meager populations), as imperiled in Maryland, and as a special concern species in Connecticut and Tennessee. It is listed as Threatened by New York State, meaning it is likely to become Endangered in the foreseeable future. Asclepias purpurascens is not federally listed and is classified as G4G5 according to the Association for Biodiversity Information, indicating that its status is not considered imperiled on a national or global scale; however, its population numbers are small throughout its range.

Threats to the species include loss and fragmentation of habitat due to development, forest succession in part from the lack of fire, and competition from invasive exotic plants. Protection and restoration of prairie remnants and oak savannas may benefit the species. Population declines are also thought to be connected to gene swapping with common milkweed and low levels of flowering.

The conservation status of Asclepias purpurascens is a significant practical consideration for its use as a dietary supplement or commercial ingredient: wild collection would be ecologically damaging where the species is rare or legally protected. No commercially cultivated supply chain for A. purpurascens-specific supplement products has been documented in the peer-reviewed or institutional literature reviewed for this article.

Common Forms and Preparations

People have used milkweed for fiber, food, and medicine all over the United States and southern Canada. Within the traditional context, preparations of Asclepias species — including closely related milkweeds — have historically taken the forms of infusions or teas made from the roots and leaves, poultices from root material applied topically, decoctions, and raw application of latex. The milky white sap was applied topically to remove warts, and the roots were chewed to cure dysentery; infusions of the roots and leaves were taken to suppress coughs and used to treat typhoid fever and asthma. No standardized commercial supplement form (e.g., encapsulated extract, tincture, or standardized powder) specific to Asclepias purpurascens has been described in the peer-reviewed scientific literature.

2. Traditional and Historical Use

Indigenous North American Traditions

Called Wah'tha or "raw medicine" by the Omaha nation, milkweed is well known to Native Americans as a cure for a variety of ailments, such as skin conditions, stomach ailments, chest pains, and the common cold. People have used milkweed for fiber, food, and medicine all over the United States and southern Canada. The ethnobotanical record pertaining to the genus Asclepias broadly, and to closely related species especially A. syriaca, is extensive; most historically documented medicinal uses of "purple milkweed" as a specific entity are indistinguishable from the broader milkweed ethnobotanical tradition due to overlapping ranges and morphological similarities between species.

A poultice of the roots was used in treating swelling, bruises, wounds, and skin ulcers; other uses included its application as an antispasmodic, to treat diarrhea and dysentery. Early European settlers called milkweed "pleurisy root" and used an infusion to relieve lung inflammation. The U.S. government listed the root of butterfly milkweed as an official herbal remedy from 1820 to 1936.

Various Native American communities have consumed milkweed for centuries — including the Tsalagi, Anishinaabe, Haudenosaunee, Lakota, Menominee, and Myaamia peoples — when prepared in a specific way. These culinary practices are very specific and result in minimal to no toxic exposure: only young plants are harvested, and they are repeatedly boiled with several changes of water, ridding the desired plant material of the water-soluble toxins.

Traditional cooking procedures of the Native American Myaamia people included repeated boiling with fresh water. The Chippewa ate milkweed to increase their appetite, to aid in milk production, and applied the roots to whistles to call deer. The Ojibwa ate the young shoots and flower buds and used the fresh flower and shoot tips to thicken meat soups.

Some Native Americans rubbed the latex juice on warts, moles, and ringworms; others drank an infusion of the rootstock to produce temporary sterility or as a laxative. One Mohawk antifertility concoction contained milkweed and Jack-in-the-pulpit, both considered dangerous and contraceptive.

Fibers from the stems of milkweed have been identified in prehistoric textiles in the Pueblo region. Tewa-speaking people of the Rio Grande still make string and rope from these fibers. At Zuni, the silky seed fibers are spun on a hand-held wooden spindle and made into yarn and woven into fabric, especially for dancers. Pueblo people ate green milkweed pods and uncooked roots from one of the species that forms fleshy tubers underground.

Because of the numerous references to medicinal uses of milkweeds in general, the genus Asclepias received its name from the Greek god of medicine and healing.

Scope and Limitations of the Ethnobotanical Record

Daniel E. Moerman's Native American Medicinal Plants is an abridged ethnobotanical dictionary detailing approximately 25,000 medicinal uses of around 2,700 plant species utilized by Native Americans across North America, serving as a resource for understanding traditional plant knowledge and medicinal practices. While this database records ethnobotanical uses for several Asclepias species, specific documentation for Asclepias purpurascens as a distinct medicinal species separate from closely related milkweeds is limited in the primary ethnobotanical literature. Much of the ethnopharmacological knowledge of North American Indians has been lost due to population destruction and displacement from their native lands by European-based settlers, though some sources of Native American ethnobotany remain.

3. Key Constituents and Active Compounds

Cardenolides (Cardiac Glycosides)

The primary and pharmacologically most significant class of secondary metabolites in Asclepias purpurascens and closely related milkweeds is the cardenolides. Cardiac glycosides are a large class of secondary metabolites found in plants. In the genus Asclepias, cardenolides in milkweed plants have an established role in plant–herbivore and predator–prey interactions, based on their ability to inhibit the membrane-bound Na⁺/K⁺-ATPase enzyme.

Cardenolides are a class of steroids (or aglycones if viewed as cardiac glycoside constituents); they are C(23)-steroids with methyl groups at C-10 and C-13 and a five-membered lactone (specifically a butenolide) at C-17. Eighteen species of Asclepias and one species of Acerates have been shown to contain cardiac glycosides with different numbers and types of cardenolides. Variation in content of cardenolides within the various tissues of a given species is also reported, meaning a single species of plant can provide a spectrum of compounds upon which insects can feed.

Cardenolide concentrations in latex and leaves are positively correlated across species, yet latex causes 27% stronger enzyme inhibition than equimolar amounts of leaf cardenolides. Three highly potent cardenolides — identified as calactin, calotropin, and voruscharin — are primarily responsible for the increased pharmacological activity of milkweed cardenolide mixtures.

All parts of milkweed plants contain toxic cardiac glycosides. The highest concentrations of cardiac glycosides are found in the plant's latex fluid, followed by the stems, leaves, and roots. The plant is most toxic just before it reaches maturity.

The related species Asclepias tuberosa (pleurisy root) contains cardenolides such as uzarigenin, coroglaucigenin, and corotoxigenin, the coumarins isorhamnetin, kaempferol, quercetin, and rutin, the steroid sitosterol, and the triterpenoids amyrin, friedelin, and lupeol. This profile is broadly representative of the phytochemistry found across the Asclepias genus.

Flavonoids and Phenolic Compounds

Studies on closely related Asclepias syriaca provide reference data for the flavonoid profile likely shared across the genus. A total of 15 phenolic compounds have been quantified in Asclepias syriaca leaf extract, including 4 flavonoids (flavonols: quercetin, rutin, and kaempferol; and the anthocyanidin delphinidin), 7 hydroxybenzoic acids, and 4 hydroxycinnamic acids. Delphinidin made the greatest contribution at 32.5 mg g⁻¹ dry extract, followed by 4-hydroxybenzoic acid, rutin, p-coumaric acid, and quercetin.

Eight flavonoids are present in the leaves of Asclepias syriaca; three have been identified as quercetin, kaempferol, and isorhamnetin. Additional flavonoid compounds isolated and identified from the seed hair of Asclepias syriaca include kaempferol, kaempferol 3-O-beta-galactopyranoside, and related kaempferol glycosides.

Triterpenes

The chloroform extract of Asclepias syriaca stem has been investigated; three triterpenes were isolated and their structures established by one- and two-dimensional NMR spectroscopy. Lupenyl acetate was isolated for the first time from A. syriaca, representing the first triterpene with a lupane skeleton described in this species; alpha-amyrin acetate and alpha-amyrin butyrate were also isolated.

Phytochemical investigation of the dried biomass of Asclepias syriaca afforded secondary metabolites showing wide structural diversity, including pentacyclic triterpenes, cardiac glycosides, flavonoid glycosides, lignans, a phenylethanoid, and a glycosylated megastigmane.

Latex Proteins and Enzymes

Four lysozymes have been purified to homogeneity from the latex of Asclepias syriaca; they have a molecular weight of approximately 28,000. The amino acid compositions of the four enzymes differ, but two appear to be related. These lysozymes have different sensitivities to the inhibitor histamine; all are relatively insensitive to N-acetyl-D-glucosamine, and two have chitinase activity. The latex also contains proteolytic enzymes that contribute to its irritant properties.

Important Note on Species-Specific Phytochemistry

It must be noted that dedicated phytochemical characterization studies published specifically on Asclepias purpurascens — as distinct from other Asclepias species — are very limited in the peer-reviewed literature searched for this article. The constituent profile described above is largely extrapolated from research on closely related congeners, particularly A. syriaca, A. tuberosa, and A. curassavica. The qualitative profile is expected to be broadly similar given shared genus membership, but quantitative differences in cardenolide content and type across species are well-documented.

4. Mechanisms of Action

Na⁺/K⁺-ATPase Inhibition

The primary and best-established mechanism of action of Asclepias cardenolides is inhibition of the sodium-potassium pump. Cardenolides are toxic when consumed in high doses because they interfere with ATPase, a protein that regulates the flux of sodium and potassium across the cell membranes of animals. Cardenolides are highly specific inhibitors of an essential ion carrier, the sodium pump.

Plants in the genus Asclepias contain cardiac glycosides in their milky latex, stems, and leaves, which inhibit the Na⁺-K⁺-ATPase enzyme found in the corneal endothelium and can cause endothelial decompensation and stromal edema. This same mechanism is central to the cardiac effects of these compounds, paralleling the action of clinically used cardiac glycosides such as digoxin.

Cardenolides vary in the structural sugar groups that conjugate to the core aglycone steroidal structures, which can alter the chemical properties of the molecule. Cardenolides can also have reactive moieties, such as aldehydes, that form hydrogen bonds between the molecule and the Na⁺/K⁺-ATPase. Not all cardenolides are predicted or shown to be equally toxic to herbivores.

Pro-Apoptotic and Anticancer Activity (Preclinical)

Calotropin — a cardenolide identified across the Asclepias family — has a similar chemical structure to cardiac glycosides such as digoxin and digitoxin. During recent years, cytotoxic and antitumor effects of cardenolide glycosides have been reported more frequently, and among cardenolides, calotropin is identified as the most promising agent. Research on related milkweed family members in cell-culture models has investigated pro-apoptotic and pro-autophagic properties: five cardenolides (calactin, calotropin, 12β-hydroxycalactin, 12β,6′-dihydroxycalotropin, and 16α-hydroxycalotropin) isolated from aerial parts of related milkweed plants have been investigated for their biological effects on a human hepatocarcinoma cell line.

Phytochemical investigation of Asclepias syriaca isolated secondary metabolites that were tested against the breast cancer cell line Hs578T; those showing IC₅₀ values lower than 50 μM were further investigated in three additional breast cancer cell lines (MCF-7, T47D, and Sk-Br-3) and the normal breast cell line Hs578Bst. This research is entirely preclinical (in vitro) and has not progressed to human clinical trials.

5. Scientific Evidence by Area of Use

5.1 Respiratory Conditions

Historically, milkweed root infusions were used for respiratory ailments including cough, bronchitis, and asthma. Early European settlers called milkweed "pleurisy root" and used an infusion to relieve lung inflammation; the U.S. government listed the root of butterfly milkweed as an official herbal remedy from 1820 to 1936. However, this official designation applied specifically to Asclepias tuberosa (butterfly weed / pleurisy root), not to A. purpurascens.

Evidence strength: There are no published clinical or human studies examining Asclepias purpurascens specifically for respiratory conditions. Evidence remains entirely ethnobotanical and historical. No controlled trials, systematic reviews, or mechanistic human data exist for this species in this indication as of the sources reviewed.

5.2 Dermatological Applications

Native Americans used the juice of milkweed as a poultice. A poultice of the roots was used in treating swelling, bruises, wounds, and skin ulcers. The latex applied directly was used for wart removal. Indigenous peoples used milkweed to treat swelling, rashes, diarrhea, and respiratory problems.

Evidence strength: All evidence is traditional and ethnobotanical. No clinical trials or controlled human studies on topical use of A. purpurascens for skin conditions were identified in the peer-reviewed literature.

5.3 Gastrointestinal Uses

Milkweed was used as an antispasmodic, to treat diarrhea and dysentery. Some Native American groups used root infusions as a laxative. Milkweed has been referenced for stomach ailments.

Evidence strength: Traditional use only. No peer-reviewed human studies exist for A. purpurascens in gastrointestinal conditions. Given the presence of cardenolides, the dose-toxicity relationship for any gastrointestinal application is poorly characterized in the modern scientific literature.

5.4 Anticancer Research (Preclinical)

Research into the anticancer potential of cardenolides found in Asclepias species has intensified in recent years. Calotropin, a highly potent cardenolide with chemical structure similar to cardiac glycosides such as digoxin and digitoxin, has been studied for cytotoxic and antitumor effects that have been reported more frequently in recent years. Researchers have aimed to analyze and discuss the specific mechanisms and molecular targets of calotropin in cancer treatment, with the goal of opening new perspectives for the adjuvant treatment of different types of cancer.

Evidence strength: All anticancer evidence for Asclepias cardenolides is in vitro (cell culture) or animal model data. No completed human clinical trials of purple milkweed or its isolated cardenolides for oncological indications were identified in the reviewed sources. Extrapolation from in vitro findings to human therapeutic efficacy cannot be made.

5.5 Cardiovascular / Diuretic Uses

Milkweed is listed in ethnobotanical literature as useful for kidney problems, dropsy (edema), and conditions of the bladder, among other uses. The structural similarity of milkweed cardenolides to digoxin implies a potential cardiotonic mechanism, but this has not been formally studied in A. purpurascens-specific human trials.

Evidence strength: No human clinical evidence for A. purpurascens specifically in cardiovascular or diuretic contexts. The cardiac glycoside content, however, makes this plant biologically plausible as a cardiovascular-active substance, with the attendant risk of toxicity at therapeutic doses.

6. Body Systems Associated With Purple Milkweed

• Cardiovascular system: Cardenolides in Asclepias species are potent Na⁺/K⁺-ATPase inhibitors with structural and pharmacological similarity to clinically used cardiac glycosides. Because it contains cardenolides, Asclepias can have digitalis-like effects and potentiate digitalis toxicity.
• Respiratory system: Historical and traditional use for cough, asthma, pleurisy, and bronchitis across the milkweed genus; early European settlers used milkweed infusion to relieve lung inflammation.
• Integumentary system (skin): Traditional topical use as poultice for swelling, wounds, warts, and skin ulcers. A poultice of the roots was used in treating swelling, bruises, wounds, and skin ulcers.
• Gastrointestinal system: Traditional use as antispasmodic and treatment for diarrhea and dysentery. The roots were chewed to cure dysentery.
• Renal/urinary system: Milkweed has been referenced for kidney problems and conditions of the bladder.
• Ocular system: Documented toxic interaction — see Safety section.

7. Dosage Forms and Reported Dosages

No standardized dosage for Asclepias purpurascens as a defined dietary supplement product has been established in any pharmacopoeia, regulatory monograph, or clinical trial identified in the sources reviewed for this article. The following reflects historical and ethnobotanical preparation descriptions only:

• Root infusion (internal): Historically prepared as a tea or decoction from dried or fresh roots for respiratory and gastrointestinal conditions; no quantified dosage is recorded in the peer-reviewed literature for A. purpurascens specifically.
• Root/leaf poultice (topical): Fresh or macerated root and leaf material applied directly to affected skin areas; no concentration or quantity specified in reviewed sources.
• Repeated boiling for food preparation: Traditional cooking procedures of the Native American Myaamia people included repeated boiling with fresh water; this procedure may reduce the concentration of cardiac glycosides in common milkweed by half. Repeated boiling prior to consumption may decrease the concentration of cardioactive steroids but should not be assumed to eliminate the risk of poisoning after ingestion, as even a modest serving of 10 g of cooked plant material could contain 1 mg of digitoxin-equivalent cardiac glycoside.

Cardiac glycosides have narrow therapeutic windows, meaning small changes in dosage can result in large differences in toxicity. No safe and effective dosage range for human use of A. purpurascens has been established in clinical studies.

8. Safety Considerations and Interactions

Systemic Toxicity from Ingestion

All parts of milkweed plants contain toxic cardiac glycosides. The highest concentrations of cardiac glycosides are found in the plant's latex fluid, followed by the stems, leaves, and roots. The plant is most toxic just before it reaches maturity. Cardiac glycosides have narrow therapeutic windows, meaning small changes in dosage can result in large differences in toxicity. When milkweed is swallowed, symptoms of toxicity usually appear within a few hours. Initial symptoms consist of stomach upset, nausea, vomiting, abdominal pain, diarrhea, weakness, lethargy, and confusion.

Symptoms of milkweed poisoning may include abdominal discomfort, nausea, vomiting, diarrhea, weakness, lethargy, and confusion, progressing to seizures, heart rhythm changes, and bradycardia. While human poisoning is rare and not well-described in the literature, milkweed ingestion can be lethal to horses and other livestock.

Drug Interactions: Digitalis and Cardiac Glycoside Medications

Because it contains cardenolides, Asclepias can have digitalis-like effects and potentiate digitalis toxicity. Interference with assays of plasma digoxin concentrations is also possible. The structure of the cardioactive steroids of the Asclepias species are closely related to digoxin, most notably the steroid ring and lactone ring. Patients receiving digoxin or related cardiac glycoside medications who ingest milkweed in any form face documented pharmacodynamic interaction risk.

Ocular Toxicity from Latex Exposure

Contact with ocular structures leads to an inflammatory response with concurrent corneal edema. Milkweed plants produce cardenolides, which are cardiac glycosides akin to digitalis capable of binding to Na⁺/K⁺-ATPases. As a result, direct ocular exposure can inhibit corneal pump function with resultant corneal edema and ocular irritation.

Corneal exposure to cardiac glycosides from milkweed plants is known to damage the endothelial sodium-potassium pumps and to cause corneal edema and decreased visual acuity. All previously documented case reports of corneal milkweed toxicity have been secondary to indirect exposure to the plant's latex. Individuals exposed to milkweed sap may experience symptoms such as eye pain, redness, tearing, foreign body sensation, and photophobia.

Fortunately, most patients recover in a matter of days with no residual sequelae. The toxic properties of milkweed sap stem from its latex, which contains irritants and proteolytic enzymes that can cause corneal epithelial damage, inflammation, and allergic reactions.

Dermal Irritation

All parts of the plant contain toxic cardiac glycosides, which can cause nausea, vomiting, diarrhea, weakness, and heart rhythm changes. Milkweed can also irritate the skin and eyes if touched.

Livestock and Animal Toxicity

A lethal dose of approximately 2% of the total body weight of fresh plant material has been estimated for livestock. Documented cases of livestock poisoning exist across multiple Asclepias species in veterinary literature.

Conservation and Legal Considerations

Purple milkweed is listed as endangered under the Massachusetts Endangered Species Act. All listed species are protected from killing, collecting, possessing, or sale and from activities that would destroy habitat and thus directly or indirectly cause mortality or disrupt critical behaviors. Similar protections apply in Wisconsin and other jurisdictions where the species is listed. Wild collection of Asclepias purpurascens from legally protected populations is thus prohibited in several states.

9. Summary of Evidence Quality

The totality of evidence relevant to Asclepias purpurascens as a dietary supplement or therapeutic agent can be characterized as follows:

• Traditional / ethnobotanical evidence: Moderate quality for the broader milkweed genus; weaker specifically for A. purpurascens as a distinct entity.
• Phytochemical evidence: Moderate for related species (A. syriaca, A. tuberosa, A. curassavica); minimal published data specific to A. purpurascens.
• Preclinical (in vitro / animal) pharmacological evidence: Exists for cardenolides across the genus, including cytotoxic, Na⁺/K⁺-ATPase inhibitory, and pro-apoptotic effects — but not specific to this species.
• Clinical / human evidence: Absent. No clinical trials, human pharmacokinetic studies, or controlled safety studies specific to Asclepias purpurascens were identified in peer-reviewed databases.
• Safety evidence: Well-documented for genus-level toxicity (cardiac glycoside poisoning, ocular toxicity, livestock lethality); sparse for species-specific data.

References

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