There is constant need for new drugs. This is because there is a lot we don't know about human biology, there are still many illnesses we cannot cure, and new illnesses appear all the time while existing medicines lose their effectiveness. New drugs are tested on animals, but because animals bodies work differently from ours, if a drug works on a caged rat, it does not mean that it will do the same for a human being. The only way to really know about a new medicine is to test it on people in a clinical trial.