In scientific studies of the effectiveness of medications there is a widely known placebo effect. This is where people experience and report effects from a non-medicine if they think they have received an actual medicine. This makes it difficult to be sure if a medicine is working because of its chemical activity or simple because people believe it is working. Most large scale drug studies, even those dealing with physical cures, typically use two groups. One group is given the medicine, the other is given a placebo such as a sugar pill. A participant in the study has no idea which they are getting. Both are tested to see if the treatment group is statistically improved over the placebo group.
Placebo drugs

For eample an analysis by NIH of 2002 study on St. John's Wort for treating depressions showed that the patients belief that they were taking the drug influenced their depression as much or more than the actual drug.

To be even more rigorous studies may use a "double blind" study in which the clinicians who are administering the medicines have no idea if they given the patients placebos or the real thing. The researchers keep track of medications using code numbers and when the study is over they can determined the effects on the groups, and who got what treatment. Studies also need to be randomized so that people are randomly assigned to treatment or control groups regardless of their condition or eagerness to participate. This makes sure that there are no self-selection biases  and that the only difference between the two groups is the medication being tested.

Even surgical treatments can be tested in this manner. Placebo patients are made to believe that they have had an actual procedure, although this is rare.

Placebo effect is an example of the mind-body connection. A person's attitude and beliefs can cause the brain to affect the body physically. Sometimes these mechanisms are well understood, while others are purely theoretical. Stress hormones may directly effect the immune system. Natural endorphins may control pain. Attitude also plays a role in a patients behavior and following healthful habits.

Placebo studies carry with them ethical considerations. If part way during a study a new medication is found to be extremely effective the study may be stopped so that the placebo patients may also get the benefit. Studies are also stopped if the treatment group has ill-effects from the medications.