Grayanotoxin is a toxin found in rhododendrons and other plants of the family Ericaceae. It can be found in honey made from their nectar and cause a very rare poisonous reaction called grayanotoxin poisoning, honey intoxication, or rhododendron poisoning. The toxin is also known as andromedotoxin, acetylandromedol, or rhodotoxin.
Grayanotoxin is a polyhydroxylated cyclic diterpene. It binds to specific sodium ion channels in cell membranes, the receptor sites involved in activation and inactivation. The grayanotoxin prevents inactivation, leaving excitable cells depolarized. Empirically the toxin is C22H36O7.
Gross physical symptoms occur after a dose-dependent latent period of minutes to three hours or so. Initial symptoms are excessive salivation, perspiration, vomiting, dizziness, weakness and paresthesia in the extremities and around the mouth, low blood pressure and sinus bradycardia. In higher doses symptoms can include loss of coordination, severe and progressive muscular weakness, bradycardia (and, paradoxically, ventricular tachycardia), and nodal rhythm or Wolff-Parkinson-White syndrome. Despite the potential cardiac problems the condition is rarely fatal and generally lasts less than a day. Medical intervention is not often needed but sometimes atropine therapy, vasopressors and other agents are used to mitigate symptoms.
Honey from Japan, Brazil, United States, Nepal, and British Columbia is most likely to be contaminated with grayanotoxins, although very rarely to toxic levels. Historically the poisoning was associated with Rhododendron luteum and Rhododendron ponticum found around the Black Sea. According to Pliny and later Strabo the locals used the honey against the armies of Xenophon in 401 BCE and later against Pompey in 69 BCE.
Pliny the Elder on Mad Honey: