Feasibility vs Safety vs Efficacy Studies

One study, one question

Health headlines often say a treatment "was tested in people." That phrase hides a big difference. Early studies ask "can this be done?" Later studies ask "is it safe?" and only large, well-controlled studies can ask "does it work?" This page explains the three kinds so you can tell which question a study actually answered. It is education, not personal medical advice. For how the labels on this site work, see how we label evidence.

The three questions

Feasibility and pilot studies

The US National Center for Complementary and Integrative Health (NCCIH) describes a pilot study as gathering information to answer "Can I do this?" rather than "Does this intervention work?" [1] Typical feasibility questions include enrollment, adherence and whether the procedures run smoothly.

NCCIH also lists common misuses. A small pilot cannot show safety: if no problems appear, investigators "cannot conclude that the intervention is safe." [1] It also cannot give a trustworthy preliminary test of benefit, because with small numbers any estimated effect size is "uninterpretable," and it could be a true result, a false positive or a false negative. [1]

For medical devices, the US Food and Drug Administration (FDA) describes an early feasibility study as a limited clinical investigation of a device early in development, typically enrolling a small number of subjects to look at the design concept, initial safety and function. [3] The FDA program is about letting early device work happen, not about showing that a device is effective.

Safety studies

In the FDA's description of drug development, Phase 1 studies usually involve 20 to 100 healthy volunteers or people with the condition, last several months, and focus on safety and dosage. [2] They are closely monitored to learn what the body does with the drug and its early side effects. Even so, the FDA reports that roughly 70% of drugs move on from this phase, which means many do not. [2]

Small safety studies can miss uncommon harms. The FDA notes that Phase 3 studies, being larger and longer, provide most of the safety data and are more likely to show long-term or rare side effects. [2] A clean early safety result is therefore a first step, not a clearance. The same agency describes the phases in more detail in clinical-trial phases explained.

Efficacy studies

Efficacy asks whether a product offers a treatment benefit in a specific population. The FDA says Phase 3 studies are designed to demonstrate this, usually with 300 to 3,000 participants and one to four years of follow-up. [2] Phase 2 studies, with up to several hundred patients, "aren't large enough to show whether the drug will be beneficial." [2]

Registration and labels do not change the answer

In the United States, the rules for registering trials on ClinicalTrials.gov treat early work differently: federal regulations define an applicable drug trial as one whose phase is other than phase 1, and an applicable device trial as one whose primary purpose is other than a feasibility study. [4] So some feasibility and phase 1 studies may never need to post results. Registration itself shows a study was planned, not that it worked; see registered trials vs published results.

Questions to ask when you see a headline

• Was this a feasibility, safety or efficacy study?
• How many people took part, and was there a comparison group?
• Did the study measure how people feel or live, or only a lab marker?
• Does the claim say "safe" or "effective" when the study was too small to show either?

Early results are a reason to watch a field, not a reason to start, stop or change a medicine or supplement. Talk with a clinician first. See also reading early-stage research.