The phases of clinical trials are the stages of testing a medicine in people: early safety, possible benefit, larger comparisons, and follow-up after marketing. The number tells us what researchers are asking. It doesn't tell us that a medicine has passed every test.

Phases of clinical trials: the FDA's plain meaning

The FDA's patient guide to clinical research describes studies in people, rather than laboratory or animal experiments. Its page carries “Content current as of: 01/04/2018.” These are its typical sizes and timelines, not fixed requirements.

Phase Who takes part Typical size Main question Typical time
1 Healthy volunteers or people with the condition 20–100 What doses can people tolerate? Several months
2 People with the condition Up to several hundred Is there an early treatment effect? Several months to 2 years
3 People with the condition 300–3,000 Does the treatment offer benefit, with acceptable risks? 1–4 years
4 People with the condition, after marketing Often several thousand; varies What else needs testing after the medicine reaches patients? Depends on the question

Why these stages exist

Researchers begin small because human effects can't be read straight from animal experiments. Larger, longer studies then test whether an early finding holds up and reveal less common side effects.

Semaglutide's STEP 1 study illustrates the scale: 1,961 adults were followed for 68 weeks in a trial published in NEJM in 2021. The STEP 1 results guide explains that particular study.

Through Promise, semaglutide and tirzepatide are dispensed as compounded medications, prepared by a pharmacy for a prescription. They are different from FDA-approved products: the formulations offered here are not FDA-approved.

As of September 30, 2026, the day this article was written, the FDA's compounding explanation states that it does not review these preparations for safety, effectiveness or quality before marketing. That page carries “Content current as of: 06/22/2026.”

Phase 1: what happens in a small group?

Phase 1 studies closely watch tolerability, meaning how people handle the drug, and what happens to it in the body. Participants may be healthy volunteers or patients, depending on the medicine.

A positive outcome means the findings support further testing at selected doses. It doesn't establish lasting benefit or settle uncommon risks.

Early CJC-1295 studies show the kind of question involved. Teichman and colleagues, JCEM, 2006, followed healthy adults for 28 or 49 days, measuring drug and hormone levels. Those measurements aren't evidence of better strength or long-term health.

Phase 2: is there enough promise to test further?

Phase 2 tests different doses in people with the condition being studied. It looks for a treatment effect while collecting more safety information.

Retatrutide's phase 2 obesity trial, NEJM, 2023, enrolled 338 adults and ran for 48 weeks. Its primary endpoint, the main measurement chosen beforehand, was weight change at week 24. It compared retatrutide with placebo, an inactive injection.

“Positive” means the study found encouraging evidence for its planned question. A bigger trial still has to examine whether that finding holds up.

What is a phase 3 trial?

Phase 3 tests the treatment question in larger groups and gathers more information about risks. A positive result generally means the study met the success rule set before it began. That is evidence about the studied treatment and population, rather than a prediction for every person.

As of September 30, 2026, the day this article was written, two recent announcements show the difference between starting phase 3 and reporting results. Zealand's September 22 release announced three petrelintide trials, planning about 7,000 participants and a main weight measurement at week 64. Lilly's September 29 release reported detailed results from retatrutide's phase 3 TRIUMPH-2 trial. Zealand also announced publication of petrelintide's phase 2 paper, in a release with a September 30 dateline. Publishing phase 2 findings doesn't turn them into phase 3 results; the petrelintide guide describes that program.

Phase 4: what remains to learn after marketing?

Phase 4 asks further questions after a medicine has marketing permission. That can mean longer safety follow-up or a comparison of treatment choices. A positive finding answers that study's question; it doesn't mean all remaining questions are settled.

One example is SURPASS-SWITCH, Annals of Internal Medicine, 2025. It followed 282 adults with type 2 diabetes for 40 weeks. Researchers compared switching to tirzepatide with increasing dulaglutide, another diabetes medicine. The main measure was blood-sugar control.

That trial was much smaller than the FDA guide's phase 4 example. Its purpose, rather than its size alone, explains the phase. It studied manufactured medicines; its findings do not establish outcomes for Promise's compounded tirzepatide preparation.

How compounded peptides relate to the phases

Compounding is a pharmacy process, not another clinical-trial phase. A routine prescription doesn't enroll someone in a trial. The active ingredient's research history and the finished pharmacy preparation are separate questions.

Some peptides have only early human studies or mainly preclinical evidence, meaning laboratory and animal work. BPC-157 has small published human studies, so calling all its evidence animal-only would be wrong.

One pilot study in Alternative Therapies in Health and Medicine, 2025, tracked two adults over three days, with BPC-157 infused into a vein on days one and two. Both had received it before. That cannot establish treatment benefit or reliably identify uncommon side effects, and it isn't equivalent to a large phase 3 trial.

BPC-157 is not FDA-approved. A licensed provider may still prescribe a compounded formulation where appropriate; that decision is between the patient and the doctor.

What the phase number does not mean

A phase 3 start isn't a result. It describes a study plan. Even a completed positive trial still needs interpretation alongside its safety findings.

First-in-human isn't a safety certificate. It means the first study of that investigational drug in people. It marks the beginning of human evidence.

Accelerated approval isn't “phase 3, but faster.” It is a distinct regulatory pathway for serious conditions with an unmet medical need. It can rely on a surrogate endpoint, a marker reasonably likely to predict a benefit. Follow-up studies must then confirm the expected benefit. The FDA's patient explanation carries “Content current as of: 02/24/2023.”

An IND, or Investigational New Drug application, is the filing that supports human testing of a new drug. An estimand describes exactly which treatment effect a study is trying to estimate. Neither term replaces the phase number; each tells us something different about the evidence.

At Promise, a licensed provider reviews every request, and not everyone qualifies. The research informs that review alongside medical history, current medicines and the formulation being considered.