After a treatment has been tested in the lab and on animals, it’s tested on people. There are four steps, or phases, of trials done with people.
Phase 1: Is the treatment safe?
A Phase 1 trial is the first time the treatment is given to people. It’s meant to find out how the treatment is safe. In particular, the researchers are concerned with bad effects which might be caused by the treatment. These range from bad breath, headaches, nausea, and vomiting to more dangerous, and even life-threatening, reactions. It’s important to find out how much of the treatment can be taken without causing serious side effects, as well as what side effects might appear. Everyone in a Phase 1 trial gets some of the treatment, but since these trials sometimes try to find out the best dose (amount) of the treatment, different people are sometimes given different doses.
A treatment in Phase 1 has not been tested on people at all, so very little is known about it. This makes Phase 1 trials riskier than Phase 2 trials. Phase 1 trials are usually three months long or less, and usually involve about a dozen people. As well as studying how a treatment is safe, they may also collect early information on how well it works.
Phase 2: Does it work?
If a Phase 1 trial finds that the treatment is safe enough, a Phase 2 trial is done. In this phase, more people are given the treatment to see whether it works at the dose figured out from Phase 1, and to study the effects more carefully. Researchers try to find out whether the treatment is effective, for example, does it raise your T4 cell count or clear up an infection? Phase 2 trials can last from a few weeks to a few months and may involve fewer than 100 people.
Phase 3: How well does it work?
If the Phase 2 trial shows that the treatment seems to work, a Phase 3 trial is started. By this time, the researchers have information about the most common side effects and the best dose to take. In a Phase 3 trial, usually hundreds or thousands of people are given the treatment to see whether it works for most people and whether it causes problems over a longer period of time. Researchers look for rare side effects that only show up in a few people or after a few years. So Phase 3 trials may go on for several years.
Combined phases
Treatments usually have to go through all three phases before they’re approved by the Health Protection Branch. But phases are sometimes combined in order to answer more than one question at a time or to speed up the approval and availability of the treatment. For example, Phase 1 and 2, or Phases 2 and 3, can be combined into one trial.
Phase 4: Post-marketing trials
Researchers do not always do post-marketing trials - trials done after the medicine is available at drug stores - but these are becoming more important now that some treatments are approved earlier than in the past. They allow for more testing over a longer period of time, to see whether any problems develop over the long term.