Understanding Clinical Trials: How New Drugs Are Tested and Monitored

Heather Mantica

06 Oct 2025 — 3 min read

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Before any new medicine can be sold, it must go through several steps of testing called clinical trials. These studies help doctors and scientists learn whether a drug is safe, whether it works, and what side effects it might cause.

What Is a Phase 1 Clinical Trial?

Phase 1 is the first time a new drug is tested in people.

These studies are small—usually involving a few dozen healthy volunteers—and they are conducted only after extensive testing in the lab and in animals. That early work helps scientists decide what dose levels are safe to begin with in humans.

The main goals of a Phase 1 trial are to:

Check safety: Make sure the drug doesn’t cause serious side effects.
Understand how the body processes the drug: Scientists study pharmacokinetics (how the body absorbs, breaks down, and removes the drug).
See how the drug affects the body: Researchers look at pharmacodynamics, which includes how the drug works and interacts with other medicines.
Find the best dose: The trial helps identify the maximum tolerated dose—the highest dose that can be taken safely.

Phase 1 trials usually test single doses first, then increase to multiple doses over short periods. Later studies may last longer as researchers gain confidence in the drug’s safety.

What Is a Phase 2 Clinical Trial?

Once a drug has passed initial safety testing, it moves into Phase 2, where the focus shifts toward finding out whether the drug actually works for the condition it’s meant to treat.

In these studies, the participants are patients who have the disease or condition, not healthy volunteers.

Phase 2 trials:

Typically include hundreds of participants.
Are designed to study both safety and early effectiveness (called “efficacy”).
Often compare the new drug to a placebo or to another treatment that’s already approved.
Use carefully controlled conditions to make results as accurate as possible.

Because these trials take place in very structured research settings, the results can show how well the drug works under ideal conditions. However, the real world can be less predictable, so later phases test the drug in broader patient populations.

What Is a Phase 3 Clinical Trial?

Phase 3 trials are much larger and longer than earlier phases. They are also called “therapeutic confirmatory” studies because they confirm that the drug is safe and effective enough for general use.

These trials typically involve thousands of patients across many hospitals or research centers. The studies are often randomized controlled trials (RCTs), meaning participants are randomly assigned to either:

The new drug, or
A control group (such as a placebo or another standard treatment).

Researchers collect detailed data on:

Effectiveness: How much better the drug works compared to the control.
Safety: What side effects occur and how severe they are.
Different patient groups: For example, people over 75 or those taking other medications.

After successful Phase 3 trials, the drug company submits all the data to regulators like the U.S. Food and Drug Administration (FDA) or the European Medicines Agency (EMA). If these agencies decide the drug’s benefits outweigh its risks, they approve it for marketing and use in patients.

What Is a Phase 4 Clinical Trial?

Even after a drug is approved, the research doesn’t stop. Phase 4 trials, also known as post-approval or “therapeutic use” studies, help scientists and regulators continue learning about the drug in real-world use.

These studies may be:

Optional: Run by drug companies or researchers who want to explore new uses or learn more about the drug’s long-term safety.
Required: Ordered by the FDA or other agencies if more safety or efficacy data are needed after approval.

Phase 4 studies help:

Track how the drug performs in broader, more diverse groups of patients.
Identify rare or long-term side effects that might not have shown up in smaller earlier trials.
Test how the drug works in combination with other treatments or in new conditions.

Sometimes, regulators may give a drug restricted approval—allowing it to be sold while requiring Phase 4 studies to confirm its safety over time.

How Do Drugs Get Monitored After They Are Approved by the FDA for Safety?

Once a drug is on the market and used by large numbers of patients, continuous monitoring becomes essential. This process is known as postmarketing surveillance.

Even the largest pre-approval studies can’t detect very rare side effects. For example, if a side effect happens in only 1 out of every 100,000 people, even a trial with several thousand participants may never see it. That’s why ongoing monitoring is crucial.

After approval, safety is tracked through:

Spontaneous reporting systems: Doctors, pharmacists, and patients can report unexpected side effects directly to the FDA.
Active monitoring programs: The FDA’s Sentinel Initiative uses electronic health records and insurance data to identify safety signals.
Additional studies: Companies may be asked to run new studies or update their product labels if new risks are found.

If a serious problem arises, the FDA can take several actions, such as: