What is a Clinical Trial Phase?

Tricia Christensen
Tricia Christensen

When clinical trials are discussed, the term clinical trial phase is often mentioned. There are generally four phases and each represents a different stage in testing. These are often most easily understood in terms of drug trials, where each clinical trial phase may mean varied kinds of tests and scope.

Clinical drug trials commonly utilize phases.
Clinical drug trials commonly utilize phases.

First, it should be mentioned that some tests have to occur before any clinical trial phase may occur, and second, the trials have to go in order from I-IV. The latter is due to the fact that later phases are more extensive in scope and certain safety levels have to be proven before progressing to extensive testing. Lastly, not all drugs enter a clinical trial phase IV testing, and it isn’t necessarily required for government approval of a medicine for sale.

Before a drug is introduced to the market, it must successfully undergo four phases of clinical trials.
Before a drug is introduced to the market, it must successfully undergo four phases of clinical trials.

Clinical trial phase I is the preliminary human testing of a medication or device. When drugs are used they are administered in very low doses, and they really aren’t even evaluated for how they might work to treat a specific disease. Instead they’re studied for human response. Information about how the body uses a drug, what kinds of side effects it causes, and what potential risks it might pose are derived from these studies.

It isn’t until clinical trial phase II that scientists begin to explore whether a drug works. Here, experiments are geared toward finding out the efficacy of a treatment. Those people enrolled in these studies may have particular conditions that a drug could purportedly treat, and since phase I work has already been done, scientists have a good sense of what side effects might occur, what dosages to give and how medications are likely to be metabolized.

Experimental medications to treat things like cancer may be tested during phase II trials. Studies are often heavily controlled, specifically to gauge effectiveness. They can take the form of double-blind placebo testing too, so that some people won’t receive a medication, even if they participate in a trial.

Potentially, the last step before a treatment is approved is a clinical trial phase III. These experiments are much wider in scope and could be conducted at many different centers across a country. Given evidence in phase II that a drug seems to work, testing it on a much larger population gathers a great deal more information about drug efficacy, dosing, side effects, and long-term effects.

With this information, many drugs are approved for sale by drug regulatory agencies, but sometimes more studies are required or the person/company that created the treatment wants to know more about it. For instance, can a treatment have beneficial effects in another illness, or do low side effects in certain areas suggest there is a better way to market a medicine? The clinical trial phase IV may answer these questions and continued study of a medication could be useful from a marketing standpoint or in some other respect.

Clinical trials can also be classed by what they study, and these may fall into different phases. For instance there are trials used for those with serious illnesses that may help to improve life quality. Other trials study prevention, look at ways to diagnose diseases, find new treatments, or find ways to screen for illnesses.

When a drug successfully completes all phases in a clinical trial, a company begins marketing the drug in that region.
When a drug successfully completes all phases in a clinical trial, a company begins marketing the drug in that region.
Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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