One group, called the study group, takes the new treatment (sometimes this group will take the new treatment in combination with the approved treatment taken by the second group). There may also be a number of study groups taking the new treatment at different doses, to show which dose works best and has the fewest side effects.
Another group, the control group, takes an approved treatment (such as AZT, ddI, ddC or) to see how it works in comparison with the new one.
Sometimes the control group takes a substance that does nothing (known as a placebo). Usually no one - not even the doctors - knows who’s taking what until the trial is over. This is called a double blind study.
Controlled trials are done to make sure that the treatment really works. If everyone in the trial gets the new treatment, there’s no way to tell if it’s the treatment making them better or if something else is doing it. They may be getting better because they’re seeing a doctor regularly, or because they’re eating better or taking better care of themselves. So a new treatment is compared to something else to see which is more effective.
Also, if people in the trial believe they’re taking a treatment that works, they may feel better, even if the treatment does not work or they are receiving a placebo. (Feeling better from taking an inactive substance, or placebo, is known as the “placebo effect.”) For example, in a trial to test a treatment to fight diarrhea, half the people who took the new treatment got better. But half the people who took the placebo got better, too. This means the treatment did not work any better than the placebo.