Cytisine, also known as baptitoxine and sophorine, is a toxic alkaloid that occurs naturally in several plant genera, such as Laburnum and Cytisus of the family Fabaceae. Despite its toxicity (LD₅₀ i.v., in mice ~2 mg/kg), it has been used medically to help with smoking cessation: it is less effective but much cheaper than similar products. Its molecular structure has some similarity to that of nicotine and it has similar pharmacological effects. Excessive doses can interfere with breathing and cause death.

Plant species that contain cytisine are found in several genera of the Faboideae subfamily of the Fabaceae family, including Laburnum, Anagyris, Thermopsis, Cytisus, Genista and Sophora. Cytisine is also present in Gymnocladus of the Caesalpinioideae subfamily.

Māmane (Sophora chrysophylla) can contain amounts of cytisine that are lethal to most animals. The palila (Loxioides bailleui, a bird), Uresiphita polygonalis virescens and Cydia species (moths), and possibly sheep and goats are not affected by the toxin for various reasons, and use māmane, or parts thereof, as food. U. p. virescens caterpillars are possibly able to sequester the cytisine to give themselves protection from getting eaten; they have aposematic coloration which would warn off potential predators.