What is the drug vincristine used for?

Vincristine is used. To treat a wide array of cancers in children and adults, including hematologic malignancies, central nervous system cancers and a host of solid tumors as well. It impairs the functioning of microtubule assemblies, such as the mitotic spindle, and therefore blocks cell division. Interesting tidbit...It was originally isolated from periwinkle (aka, vinca), hence its designation as a vinca alkaloid.
Lots of uses. Used in multiple malignancies but also used in non-malignant hematologic (blood) disorders, such as itp (immune mediated thrombocytopenia). This is low platelets. Hope this helps.