GLP-1 Clinical Studies Explained: How to Read the Research

Clinical trial results for GLP-1 medications are cited constantly in media coverage, patient discussions, and prescribing conversations — but the numbers are rarely explained in terms of what they actually mean for a given individual. This guide breaks down the major trials and the statistical concepts behind them.

How to read a weight loss trial

The key metrics in a weight loss trial are:

• Mean % body weight loss — the average weight loss across all participants. This is what's most commonly cited but is the least informative for any individual.
• Responder analysis — what percentage of participants achieved ≥5%, ≥10%, ≥15% weight loss. This shows the distribution better than the mean.
• Placebo-subtracted loss — the difference between drug and placebo. Placebo arms in these trials lose 2–4% body weight on average (from lifestyle counseling alone), so the drug effect is always the difference, not the absolute number.
• Withdrawal rates and reasons — discontinuation due to side effects indicates tolerability. High dropout rates can make a drug look more effective than it is (completers are self-selected).
• Intent-to-treat vs. per-protocol — ITT analysis includes all randomized participants; per-protocol only includes those who completed. ITT is more conservative and more realistic.

The STEP trial program (semaglutide 2.4mg)

The STEP program tested semaglutide 2.4mg weekly (Wegovy) across five major trials:

• STEP 1 (N=1961): Adults without diabetes, BMI ≥30 or ≥27 with comorbidity. Mean weight loss: 14.9% (semaglutide) vs. 2.4% (placebo) at 68 weeks. 69.1% of semaglutide participants lost ≥10% of body weight vs. 12% on placebo.
• STEP 2 (N=1210): Adults with type 2 diabetes. Mean loss: 9.6% vs. 3.4% at 68 weeks.
• STEP 5 (N=304): Two-year extension. Weight loss maintained at ~15% through 104 weeks, establishing durability with continued use.

For full detail on individual variation in outcomes, see How Much Weight Can You Lose on Semaglutide.

The SURMOUNT trial program (tirzepatide)

Tirzepatide's SURMOUNT program tested the dual GLP-1/GIP agonist at doses up to 15mg weekly. SURMOUNT-1 (N=2539) showed:

• Mean weight loss: 20.9% at 15mg dose vs. 3.1% on placebo at 72 weeks
• 89.5% of participants at 15mg achieved ≥5% weight loss; 56.7% achieved ≥20%
• Mean absolute weight loss: approximately 22.5 kg (49.5 lbs) at 15mg

These are the largest weight loss numbers ever recorded in a randomized pharmaceutical trial. For how tirzepatide's dual mechanism produces these results, see How Tirzepatide Works. For a side-by-side comparison with semaglutide, see Semaglutide vs. Tirzepatide.

The SCALE trial program (liraglutide)

Liraglutide (Saxenda), an earlier GLP-1 receptor agonist requiring daily injection, was studied in the SCALE program. SCALE Obesity and Prediabetes (N=3731) showed 8% mean weight loss vs. 2.6% on placebo at 56 weeks. This is notably lower than semaglutide results, which explains the shift in clinical practice toward semaglutide and tirzepatide.

What the trials don't tell you

All major GLP-1 trials involve significant lifestyle counseling alongside medication — creating a confound that makes it impossible to isolate the pure drug effect from the behavioral support effect. Real-world outcomes, without structured counseling, may differ from trial results. Trials also exclude many real-world populations and have dropout rates of 5–15%, which affects generalizability. For a look at what the evidence says about long-term outcomes, see Sustainable Weight Management Research.

A clinical trial tells you the average effect in a carefully selected population under controlled conditions. Your individual response will vary — but the magnitude of these effects is large enough that it matters for most people who complete treatment.