An Erowid reader comments:
"[Periwinkle] is used to make a couple of very immunosuppressive drugs (vincristine and vinblastine) and will have these same effects on those that use the whole plant, even if used occasionally. This should probably be written ... as a warning so that no one decides to use periwinkle regularly. It is probably a bad idea to use it at all, given how strong vinblastine in particular is, but at least make sure people know the risks."
"If you look at the alkaloid content, you will find both vincristine and vinblastine. If you look up those, you will find they are both chemotherapy for suppression of lymphocytes. Just search for vincristine and madagascar periwinkle. I do not know how toxic they are with ingestion, but they have been proven to decrease white cell counts, and both are natural alkaloids.
"The plant's therapeutic uses came to the attention of Canadian and American medical researchers during World War II when they learned that soldiers stationed in the Philippines used Madagascar Periwinkle leaves to substitute for unavailable insulin. During the 1950's, while investigating the effects of Madagascar Periwinkle extracts, the researchers found them to have no appreciable effect on blood sugar levels, but they did reduce the white blood cell count in laboratory animals without significant side effects. More recently, 2 alkaloids in Madagascar Periwinkle leaves, vinblastine and vincristine, were identified as active anti-cancer agents that could be used in chemotherapy. Vinblastine is used for patients with Hodgkin's disease and vincristine is used for children with leukemia. With the introduction of vincristine, the survival rate for children with leukemia jumped from 20 to 80 percent."
The use of Periwinkle in immunosuppressive drugs is reviewed by "Studies on the Suppression of Immune Response by the Periwinkle Alkaloids Vincristine and Vinblastine", Aisenberg and Wilkes, 1964.