Justicia gendarussa, commonly known as Willow-leaved justicia (Marathi: बाकस, bakas, काळा अडुळसा, kala adulasa; Sanskrit: कसनः, kasanah, वैध्यसिंहा, vaidyasinha), Lapsulis in Creole Seychelles; is a small erect, branched shrub. According to Plants of the World Online it is native to the Indian subcontinent (India, Bangladesh, Sri Lanka, and Bhutan), Indochina, Peninsular Malaysia, Sumatra, the Philippines, and New Guinea, and has been introduced to Nepal, the western Himalayas, Pakistan, southern China including Hainan and Taiwan, eastern Africa, and the Mascarene Islands, Comoro Islands, and Seychelles. It has been described as rare and endemic to India, though those claims are at least confusing, in the context of statements that the plant is widely used in various forms for many of its medicinal and insecticidal properties, and that it is a quick-growing, evergreen forest shrub considered to be a native of China and distributed in Sri Lanka, India and Malaysia.

J. gendarussa is harvested for its leaves for the treatment of various ailments. It is said to be useful for the treatment of asthma, rheumatism and colics of children. Used as a treatment for skin problems like eczema. It may have the potential to be the basis for a birth control pill for men. Clinical tests are being conducted in Indonesia.

The plant has shown promise as a source of a compound that inhibits an enzyme crucial to the development of HIV.

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