The first medical trial is often cited as being carried out by James Lind who in 1747 gave sailors with scurvy different diets to see what foods had an impact. He discovered citrus fruits cured the illness. Since then drug trials have become much more rigorous and are our current best way to trial new drugs.
The gold standard of drug trials is the double blind randomly-controlled trial. This means both the experimenter and the patient don’t know if they are on the active drug or a placebo and the patients have been randomly sorted into which groups take which treatment. These methods are the best way to create a trial which only tests the drug itself and isn’t influenced by other factors.
Without this blinding, many trials and tests would be open to personal bias or other unconscious errors creeping in and skewing the results.