Vincristine is a chemotherapy drug, primarily used to treat certain types of leukemia. It can also be used as an immunosuppressant to treat thrombotic thrombocytopenic purpura and other similar conditions. It's derived from a species of the periwinkle plant.
Vincristine works by blocking a key protein involved in the separation of chromosomes during cell division. Instead of multiplying, the cell instead undergoes apoptosis, essentially self-destructing itself.
Side effects are typical of chemotherapy agents that prevent cell division, such as hair loss, peripheral neuropathy, constipation, and headaches.