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The Asclepias genus is a group of perennial flowering herbs also known as milkweed due to their milky sap. There are over 200 species in the Asclepias genus that are native to Africa, North America, and South America. Monarch butterflies cannot survive without milkweed; their caterpillars only eat milkweed plants, and adult monarchs need milkweed to lay their eggs. The plant is most easily identified by its distinctive pod-like fruit containing densely packed seeds. When the fruit pods mature and turn brown, they burst and release the seeds. All parts of the plant contain toxic cardiac glycosides. The highest concentrations of cardiac glycosides are found in the sap, followed by the stems, leaves, and roots. The plant is most toxic just before it reaches maturity.
Cardiac glycoside-containing plants have been used since ancient times for medical purposes. Milkweed has been used by indigenous peoples for swelling and rashes, diarrhea, and respiratory issues. However, cardiac glycosides have narrow therapeutic windows, meaning small changes in dosage can result in large differences in toxicity. Due to the prevalence of the plant, exposure to milkweed plants is very common. From 2011 through 2015, Poison Control received more than 7,000 calls regarding cardiac glycoside-containing plants.
When swallowed, symptoms of milkweed toxicity usually appear within a few hours. Initial symptoms consist of stomach upset, nausea, vomiting, abdominal pain, diarrhea, weakness, lethargy, and confusion. Severe toxicity includes seizures, heart rhythm changes, and severe slowing of the heart rate. Milkweed can cause coma and respiratory paralysis, which can lead to death.
The sap from the plant can also cause skin and eye irritation. If your skin comes into contact with milkweed sap, you should immediately wash the affected area thoroughly with soap and water. If someone experiences eye irritation after getting sap in their eye, gently rinse the eye with room temperature water for 10–15 minutes. If there are any persistent skin or eye pain or symptoms, seek medical examination and treatment.
If you suspect someone has been exposed to milkweed and is having a problem, check the webPOISONCONTROL® online tool for guidance or call Poison Control at 1-800-222-1222.
Diana Pei, PharmD
Certified Specialist in Poison Information
References
Butterfly milkweed Asclepias tuberosa L. Washington; US Department of Agriculture; 2006 May 31 [cited 2020 Dec 1].
Common milkweed Asclepias syricacia F. asclepidaceae. Fall wildflowers of New England. Waltham (MS): Brandeis University; [cited 2020 Dec 1].
Hack JB. Cardioactive steroids. In: Nelson LS, Howland MA, Lewin NA, Smith SW, Goldfrank L, Hoffman RS, editors. Goldfrank's toxicologic emergencies. 11th ed. New York: McGraw Hill; 2019.
Nelson CJ, Seiber JN, Brower LP. Seasonal and intraplant variation of cardenolide content in the California milkweed, Asclepias eriocarpa, and implications for plant defense. J Chem Ecol. 1981 Nov;7(6):981–1010.
Plants–cardiac glycosides. In: IBM Micromedex POISINDEX System [Internet database]. Greenwood Village (CO): IBM Watson Health; c2020 [cited 2020 Nov 29].