Chemotherapy is a general term for treatments that use chemical agents (drugs) to kill cancer cells. Many different kinds of drugs are used, either alone or in combination, to treat different cancers. The specific drug or combination used is chosen to best combat the type and extent of cancer present. Chemotherapy drugs are tested against various forms of cancer in an effort to find out which drugs work against that particular type of cancer. Multiple drugs, each individually effective against a certain cancer, are often combined to try and maximize the effect against the cancer. Drugs are combined so that there are few overlapping side effects, to make the treatment more tolerable. These combinations are then tested in clinical trials to see how effective they are. If a combination works better than the current "standard" treatment, it will become the new standard therapy.