Docetaxel is a chemotherapy drug. It is used to treat cancerous tumors of the breast, head, neck, stomach, prostate and lungs. Although it is often felt to be more potent than the similar drug paclitaxel, clinical studies have shown the effectiveness of each drug is nearly identical, but docetaxel has more severe side effects. Moreover, it appears tumors develop a tolerance to docetaxel more quickly. However, one of it's benefits is that it only has to be administered no more than once a week and, in many cases, only once every three weeks.