The plant genus responsible for “mad honey” poisoning containing grayanotoxin is Rhododendron. Several Rhododendron species produce grayanotoxins, chemicals that can pass from nectar and pollen into honey collected by bees.
Mad honey is especially associated with the Black Sea region of Turkey and the Himalayan region of Nepal. In Turkey, Rhododendron luteum and Rhododendron ponticum are important sources, and the honey is often called deli bal. The Gurung people of Nepal have traditionally harvested it using Himalayan giant honey bees.
Grayanotoxins interfere with voltage-gated sodium channels, keeping them abnormally open. This can cause dizziness, nausea, tingling, low blood pressure, slow heart rate, fainting, and, in severe cases, seizures or dangerous heart rhythms. Bees themselves are generally unaffected by the toxin.
A common mix-up is assuming that all honey from a Rhododendron-growing area is poisonous. Toxicity depends on the species, the amount of grayanotoxin in the flowers, and how concentrated or diluted the honey becomes. Other Ericaceae genera can also contain grayanotoxins, but Rhododendron is the classic and canonical source associated with mad honey.