placebo

The mind n body are very powerful. What is known as the placebo effect influences all drug trials, making them essentially invalid. Once you know what a drug is being tested for, the expectation that it will be effective influences the result. The presence of side-effects assures that the person knows they are on the actual drug, eliminating the blinding that is supposed to be present. The only way around this is to not tell the person what condition or symptom the drug is even intended to treat.