GLP-1 Clinical Trial Data: What the Numbers Actually Mean
📊 Why This Matters
Every GLP-1 ad cites clinical trial data. "15% body weight loss!" "20% cardiovascular risk reduction!" But those numbers are averages that obscure enormous individual variation. This guide teaches you to read the data the way researchers do — so you can set realistic expectations and choose the right medication.
The Trials That Matter
Before we decode the numbers, here are the major trial programs you'll see cited:
| Trial Program | Drug | What It Tested |
|---|---|---|
| STEP 1–5 | Semaglutide 2.4mg (Wegovy) | Weight loss in obesity |
| SELECT | Semaglutide 2.4mg (Wegovy) | Cardiovascular outcomes |
| SURMOUNT 1–5 | Tirzepatide (Zepbound) | Weight loss in obesity |
| OASIS 1–4 | Oral semaglutide 25mg (Wegovy pill) | Weight loss, oral format |
| TRIUMPH 1–7 | Retatrutide (investigational) | Weight loss, diabetes, liver, CV |
| ATTAIN 1–4 | Orforglipron (investigational) | Weight loss, oral non-peptide |
Concept 1: "Average Weight Loss" Doesn't Mean YOUR Weight Loss
When you read "STEP 1 showed 15% average body weight loss," that's the mean across the entire study population. But averages hide the distribution:
- In STEP 1, roughly one-third of participants lost ≥20% of their body weight
- About 86% lost at least 5%
- But roughly 14% didn't achieve even 5% loss
The same pattern holds for SURMOUNT-1 (tirzepatide): the 22.5% average includes people who lost 30%+ and people who lost very little. Your individual response depends on genetics, starting weight, dose tolerance, diet, activity level, and adherence.
What this means for you: Don't anchor your expectations to the headline number. A realistic range is roughly half to double the average — so for semaglutide, expect somewhere between 8% and 25% body weight loss over 12-18 months. If you're not in the upper range, that doesn't mean the drug "isn't working."
Concept 2: ITT vs. Completer Analysis
This is the single most misunderstood distinction in clinical trial reporting, and it can make a drug look dramatically more or less effective.
Intent-to-treat (ITT): Includes everyone who was randomized into the trial, whether or not they finished it. If someone dropped out in month 2, their last recorded weight is carried forward.
Completer analysis (or "treatment policy estimand"): Only includes people who took the medication for the entire study duration. People who quit are excluded from the calculation.
The difference is huge. In STEP 1:
- ITT analysis: ~14.9% average weight loss
- Completer analysis: ~16.9% average weight loss
Both numbers are accurate — they're just measuring different things. ITT reflects "what happens to everyone who starts," including people who stop early (often because of side effects or lack of motivation). Completer analysis reflects "what happens to people who stick with it."
Which one matters more? If you're trying to decide whether to start treatment, the ITT number is more realistic — it accounts for the possibility that you might not tolerate the medication. If you're already on treatment and tolerating it well, the completer number is a better predictor of your outcome.
Concept 3: Placebo-Subtracted vs. Absolute Weight Loss
Clinical trials always include a placebo group. Some of those placebo participants lose weight too — from the lifestyle counseling, diet changes, and heightened awareness that comes from being in a study. The question is: how much weight loss is actually from the drug?
In STEP 1, the placebo group lost about 2.4% of body weight. The semaglutide group lost 14.9%. So:
- Absolute weight loss (drug group): 14.9%
- Placebo-subtracted: 14.9% − 2.4% = 12.5%
Drug companies and media almost always report the absolute number (14.9%) because it's bigger and more impressive. Researchers care more about the placebo-subtracted number because it isolates the drug's actual contribution.
What this means for you: In the real world, you won't be getting the structured lifestyle counseling that trial participants receive. Your results might actually be somewhat different from either number. But the absolute number is probably the more useful reference point for practical planning.
Concept 4: What Trial Duration Tells You
Weight loss in GLP-1 trials typically follows a predictable curve:
- Months 1-4: Rapid weight loss during dose titration
- Months 4-12: Continued weight loss as you reach and maintain therapeutic dose
- Months 12-18: Weight loss plateaus and stabilizes
- Beyond 18 months: Weight maintenance phase
Most major trials run 68-72 weeks (~16-18 months), which captures the full loss curve. When comparing medications, make sure you're comparing results at similar time points. Comparing a 72-week result for one drug to a 48-week result for another isn't apples to apples.
The Major Trial Results, Decoded
| Trial | Duration | Avg. Weight Loss | ≥10% Loss | ≥20% Loss |
|---|---|---|---|---|
| STEP 1 (Wegovy) | 68 weeks | 14.9% | ~69% | ~32% |
| SURMOUNT-1 (Zepbound 15mg) | 72 weeks | 22.5% | ~87% | ~57% |
| SURMOUNT-5 (Zepbound vs Wegovy) | 72 weeks | 20.2% vs 13.7% | — | — |
| OASIS 4 (Wegovy pill) | 68 weeks | ~16.6% | ~70% | ~35% |
| TRIUMPH-4 (Retatrutide*) | 68 weeks | 28.7% | ~93% | ~72% |
| ATTAIN-1 (Orforglipron*) | 72 weeks | 12.4% | ~55% | — |
*Investigational — not FDA-approved
Concept 5: The Cardiovascular Data Changes Everything
The SELECT trial was a game-changer — not for weight loss data, but for cardiovascular outcomes. It showed that semaglutide 2.4mg (Wegovy) reduced the risk of major adverse cardiovascular events (heart attack, stroke, or cardiovascular death) by 20% in people with established cardiovascular disease and obesity — regardless of diabetes status.
This is the single most important piece of data for insurance coverage. It's why Wegovy has a cardiovascular indication, why Medicare's GLP-1 Bridge program includes it, and why many insurers have added coverage for Wegovy when they won't cover other GLP-1s for weight loss alone.
Zepbound (tirzepatide) does not yet have cardiovascular outcomes data — that trial is still running. This is a critical gap: tirzepatide produces more weight loss, but semaglutide has proven cardiovascular protection. For patients with heart disease risk factors, this data could make Wegovy the better choice despite lower average weight loss.
Concept 6: What Happens When You Stop
This is the question no one likes to answer: what happens when you discontinue GLP-1 medication?
STEP 1 extension data showed that participants who stopped semaglutide regained approximately two-thirds of their lost weight within one year of discontinuation. The SURMOUNT-4 trial showed similar patterns for tirzepatide.
This doesn't mean the medications "don't work" — it means obesity is a chronic condition that, for most people, requires ongoing treatment. Just like stopping blood pressure medication leads to blood pressure returning, stopping GLP-1 medication leads to weight returning. The medical consensus increasingly treats GLP-1 therapy as long-term or lifelong, not a short-term intervention.
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Now that you understand what the numbers mean, here's the practical framework:
Look at the ≥10% threshold, not just the average. Clinically meaningful health improvements (reduced blood pressure, improved insulin sensitivity, cardiovascular risk reduction) begin at approximately 5-10% weight loss. The percentage of trial participants achieving ≥10% tells you your probability of a clinically significant response — and for all currently approved GLP-1 medications, that number is above 50%.
Compare at similar time points. A 72-week result isn't comparable to a 24-week result. When comparing medications, always check the trial duration.
Weigh efficacy against tolerability. The most effective medication is the one you'll actually take consistently. If injectable Zepbound produces more weight loss than the Wegovy pill but you won't take injections, the pill is the better choice for you.
Factor in the cardiovascular data. If you have heart disease risk factors, Wegovy's SELECT trial data provides a level of evidence that other GLP-1s haven't matched yet. That matters for both your health and your insurance coverage.
Plan for the long term. GLP-1 trial data consistently shows weight regain after discontinuation. Choose a medication and provider that you can sustain — financially and logistically — for the long haul.