Paclitaxel is a taxoid chemotherapy drug originally extracted from the bark of the Pacific yew. It is sold under the trade name Taxol. It works against cancer by interfering with mitosis. It binds to microtubules and inhibits their depolymerization (molecular disassembly) into tubulin. This means that paclitaxel blocks a cell's ability to break down the mitotic spindle during mitosis (cell division). With the spindle still in place the cell can't divide into daughter cells (this is in contrast to drugs like colchicine and the Vinca alkaloids, which block mitosis by keeping the spindle from being formed in the first place).

Paclitaxel is given intravenously (it irritates skin and mucous membranes on contact), and is most effective against ovarian cancers (carcinomas) and advanced breast cancers. Slightly less than half of the patients receiving paclitaxel during clinical trials developed an allergic reaction to it, which resulted in problems ranging from rashes or a mild drop in blood pressure to major breathing problems, hives and/or fluid buildup around the heart. To counteract this risk of developing a reaction, patients are pre-treated with corticosteroids and diphenhydramine (an antihistamine) before they undergo paclitaxel therapy. Other adverse reactions include an abnormally low neutrophil count (which can leave the patient vulnerable to infection) and abnormally low platelet counts, which can cause hard-to-control bleeding. Anemia and bone and muscle pain were also common side effects.