Calotropis is a genus of flowering plants in the dogbane family, Apocynaceae. They are commonly known as milkweeds because of the latex they produce. Calotropis species are considered common weeds in some parts of the world. The flowers are fragrant and are often used in making floral tassels in some mainland Southeast Asian cultures. Fibers of these plants are called madar or mader. The plant is known as aak in Ayurveda and was used in cases of cutaneous diseases, intestinal worms, cough, ascites, asthma, bronchitis, dyspepsia, paralysis, swellings, intermittent fevers, anorexia, inflammations and tumors. In large doses, Arka is known to act as a purgative and an emetic.  
The milky exudation from the plant is a corrosive poison. The latex is said to have mercury-like effects on the human body, and is some times referred to as vegetable mercury and is used in place of mercury in aphrodisiacs. It is used variously but sometimes leaves are fried in oil for medicinal purposes.
Calotropis species are usually found in abandoned farmland. Cattle often stay away from the plants because of their unpleasant taste and their content of cardiac glycosides.
Root bark has a Digitalis-like effect on the heart, but was earlier used as a substitute of ipecacuanha.
They are poisonous plants; calotropin, a compound in the latex, is more toxic than strychnine. Calotropin is similar in structure to two cardiac glycosides which are responsible for the cytotoxicity of Apocynum cannabinum. Extracts from the flowers of Calotropis procera have shown strong cytotoxic activity in the patients of colorectal cancer. They are harmful to the eyes.
C. gigantea and C. procera are the two most common species in the genus. C. gigantea grows to a height of 8 to 10 ft (2.4 to 3.0 m) while C. procera grows to about 3 to 6 ft (0.91 to 1.8 m). The leaves are sessile and sub-sessile, opposite, ovate, cordate at the base. The flowers are about 1.5 to 2 in (3.8 to 5.1 cm) in size, with umbellate lateral cymes and are colored white to pink and are fragrant in case of C. procera while the flowers of C. gigantea are without any fragrance and are white to purple colored, but in rarer cases are also light green-yellow or white. The seeds are compressed, broadly ovoid, with a tufted micropylar coma of long silky hair. 
The stigmas and androeciums are fused to form a gynostegium. The pollen are enclosed in pollinia (a coherent mass of pollen grains). The pollinia are attached to an adhesive glandular disc at the stigmatic angle. When a bee lands on one of these, the disc adheres to its legs, and the pollinium is detached from the flower when the bee flies away. When the bee visits another flower, the flower is pollinated by the adhering pollinium on the bee.
- As mentioned earlier in the article, the plant was described as Aak in Ayurveda and finds its use in many traditional medicines.
- The flowers of the plant are offered to the Hindu deity Shiva and Hanuman.
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- "Calotropis R. Br.". Germplasm Resources Information Network. United States Department of Agriculture. 2003-03-13. Retrieved 2010-06-26.
- Puri, H. S. (2003) RASAYANA: Ayurvedic Herbs for Longevity and Rejuvenation. Taylor & Francis, London.
- S. Morris Kupchan, John R. Knox, John E. Kelsey, and J. A. Saenz Renauld: Calotropin, a Cytotoxic Principle Isolated from Asclepias curassavica L. Science 25 December 1964.