Clinical trials need to be carried out under carefully controlled conditions in order not to come to the wrong conclusions and jeopardize patients' safety. In a blind trial, the best method of testing, the patients do not know whether they are being given the drug or a placebo. A placebo is something that looks like the drug but actually contains no active ingredients. In some trials, neither the doctors nor the patients know who has taken what until the trial is over. This is a very good way of filtering out any errors.