Clinical trials are a way to test new methods of diagnosing, treating, or preventing health conditions. The goal is to determine whether something is both safe and effective.

A variety of things are evaluated through clinical trials, including:

  • medications
  • medication combinations
  • new uses for existing medications
  • medical devices

Before doing a clinical trial, investigators conduct preclinical research using human cell cultures or animal models. For example, they might test whether a new medication is toxic to a small sample of human cells in a laboratory.

If the preclinical research is promising, they move forward with a clinical trial to see how well it works in humans. Clinical trials happen in several phases during which different questions are asked. Each phase builds on the results of previous phases.

Keep reading to learn more about what happens during each phase. For this article, we use the example of a new medication treatment going through the clinical trial process.

Phase 0 of a clinical trial is done with a very small number of people, usually fewer than 15. Investigators use a very small dose of medication to make sure it isn’t harmful to humans before they start using it in higher doses for later phases.

If the medication acts differently than expected, the investigators will likely to do some additional preclinical research before deciding whether to continue the trial.

During phase I of a clinical trial, investigators spend several months looking at the effects of the medication on about 20 to 80 people who have no underlying health conditions.

This phase aims to figure out the highest dose humans can take without serious side effects. Investigators monitor participants very closely to see how their bodies react to the medication during this phase.

While preclinical research usually provides some general information about dosing, the effects of a medication on the human body can be unpredictable.

In addition to evaluating safety and ideal dosage, investigators also look at the best way to administer the drug, such as orally, intravenously, or topically.

According to the FDA, approximately of medications move on to phase II.

Phase II of a clinical trial involves several hundred participants who are living with the condition that the new medication is meant to treat. They’re usually given the same dose that was found to be safe in the previous phase.

Investigators monitor participants for several months or years to see how effective the medication is and to gather more information about any side effects it might cause.

While phase II involves more participants than earlier phases, it’s still not large enough to demonstrate the overall safety of a medication. However, the data collected during this phase helps investigators come up with methods for conducting phase III.

The FDA estimates that about of medications move on to phase III.

Phase III of a clinical trial usually involves up to 3,000 participants who have the condition that the new medication is meant to treat. Trials in this phase can last for several years.

The purpose of phase III is to evaluate how the new medication works in comparison to existing medications for the same condition. To move forward with the trial, investigators need to demonstrate that the medication is at least as safe and effective as existing treatment options.

To do this, investigators use a process called randomization. This involves randomly choosing some participants to receive the new medication and others to receive an existing medication.

Phase III trials are usually double-blind, which means that neither the participant nor the investigator knows which medication the participant is taking. This helps to eliminate bias when interpreting results.

The FDA usually requires a phase III clinical trial before approving a new medication. Due to the larger number of participants and longer duration or phase III, rare and long-term side effects are more likely to show up during this phase.

If investigators demonstrate that the medication is at least as safe and effective as others already on the market, the FDA will usually approve the medication.

Roughly of medications move on to phase IV.

Phase IV clinical trials happen after the FDA has approved medication. This phase involves thousands of participants and can last for many years.

Investigators use this phase to get more information about the medication’s long-term safety, effectiveness, and any other benefits.

Clinical trials and their individual phases are a very important part of clinical research. They allow the safety and effectiveness of new drugs or treatments to be properly assessed before being approved for use in the general public.