The treatment being tested may be an entirely new drug added to existing combinations. Or it could be a new way of using existing drugs, such as comparing once-daily dosing versus twice-daily dosing.

Randomisation

This is an essential feature of almost all clinical trials. Everyone who joins the trial will be assigned to one of the treatment groups at random, usually by computer. This is the best way to make sure that people in different arms of the trial are broadly similar, so that reliable comparisons can be made between the effects of the treatments used in the different trial arms.

Blinded studies

In some trials, neither the researchers nor the participants are supposed to know who is getting the drug and who is getting the standard treatment or no treatment. (The medications are all 'packaged' to look the same.) This is called double-blinding (because both researcher and participant are said to be 'blind' to who is receiving what). When only the participants do not know which treatments they are receiving, but the researchers do, then the clinical trial is known as single-blind.

The purpose of blinding is to make sure that nobody's expectations affect the results of the trial. A doctor who knows which patients were getting a particular treatment might unconsciously assess or treat them in a different way. Similarly, participants who know their treatment might describe their symptoms differently, or they might choose to drop out.

Comparison trial

The most common type of clinical trial is a comparison trial. This is a trial in which one group of people who receive a new treatment is compared with another group of people who receive an existing or standard therapy. When there is a recognised standard treatment for a particular condition, then trials will be designed to compare an experimental drug with that standard treatment. In this case it would be unethical to give some trial participants no treatment rather than the standard treatment.

Placebo-controlled trials

Where no standard treatment exists for a disease, a trial may compare a new treatment with a dummy drug called a placebo. None of the participants will know whether they are receiving the new drug or the placebo.

Dose comparison trial

Other comparison trials may simply compare different doses of the same drug.

Open label or uncontrolled studies

Trials in which treatment allocation is not blinded are also called ‘open label’. This means that everyone in the trial, and their doctors, know exactly which treatments they are taking. Some studies will evaluate the effectiveness and safety of a new drug without a control group. This means that everyone who enters the study receives the study medication.