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Medically reviewed byDr. Rachel Morrison, MD, ABOM·Updated July 2026
Research

What the clinical trials actually show

GLP-1 receptor agonists are the most studied class of weight-loss medications in history. Over 45,000 participants across dozens of randomized controlled trials. Here's what the evidence reveals - the results, the limitations, and what it means for your treatment.

Start your treatment →How GLP-1s work
The Big Picture

Evolution of GLP-1 obesity treatments

From the first GLP-1 approvals to today's triple agonists, each generation has produced significantly better outcomes. Here's how the science has progressed.

Liraglutide

2014 - Saxenda
-8%

First GLP-1 for obesity. Daily injection. SCALE trials. Proof of concept that GLP-1 pathway could treat obesity.

Semaglutide

2021 - Wegovy
-16.9%

Weekly injection. STEP program. Doubled the efficacy of liraglutide. First GLP-1 with CV outcomes data (SELECT).

Tirzepatide

2023 - Zepbound
-22.5%

First dual agonist (GLP-1 + GIP). SURMOUNT program. Lower nausea. Approaching bariatric surgery territory.

Retatrutide

Phase 3 - TRIUMPH
-28.6%

First triple agonist (+ glucagon). Increases energy expenditure. Weight loss not yet plateaued at 48 weeks.

Pivotal Trials

Major trial programs at a glance

The landmark randomized controlled trials that established GLP-1 therapy as the new standard of care for obesity.

Semaglutide

STEP 1

The pivotal trial: 1,961 adults, semaglutide 2.4 mg vs placebo, 68 weeks. Lifestyle intervention in both arms.

-16.9%weight loss
Placebo: -2.4%. 32% of semaglutide group lost ≥20%. Only 7% discontinued due to side effects. Published: NEJM, Feb 2021.
Semaglutide · Cardiovascular

SELECT

Cardiovascular outcomes: 17,604 adults with overweight/obesity + established CV disease. Semaglutide 2.4 mg vs placebo. Median 39.8 months.

-20%MACE reduction
First obesity drug to show cardiovascular mortality benefit independent of diabetes status. Published: NEJM, Nov 2023. Landmark.
Semaglutide · Long-term

STEP 5

Longest semaglutide trial: 304 adults, 104 weeks (2 years). Assessed durability of weight loss beyond 1 year.

-15.2%sustained at 2 years
Weight loss maintained from year 1 to year 2 with continued treatment. Placebo: -2.6%. Confirms long-term efficacy does not wane.
Tirzepatide

SURMOUNT-1

Pivotal: 2,539 adults, tirzepatide 5/10/15 mg vs placebo, 72 weeks. No diabetes. The largest weight loss in a pivotal obesity trial at the time.

-22.5%weight loss (15 mg)
39.7% of the 15 mg group lost ≥25% body weight. Lower nausea rate than semaglutide (~26% vs ~44%). Published: NEJM, July 2022.
Tirzepatide · T2D

SURMOUNT-2

938 adults with T2D and obesity. Tirzepatide 10 and 15 mg vs placebo. 72 weeks. First large dual-agonist trial in diabetic obesity.

-14.7%weight loss (15 mg)
Placebo: -3.2%. Also reduced HbA1c by 2.1 percentage points. Diabetes typically blunts GLP-1 weight loss - 14.7% in T2D is exceptional.
Retatrutide

Phase 2

338 adults with obesity. Retatrutide 1/4/8/12 mg vs placebo. 48 weeks. First triple-agonist trial in humans.

-28.6%weight loss (12 mg)
Weight curve still declining at 48 weeks - no plateau. 83% lost ≥15%. 63% lost ≥20%. Published: NEJM, Aug 2023. Phase 3 (TRIUMPH) now enrolling.
Side-by-Side

Cross-trial comparison

Key endpoints compared across compounds. Important caveat: these are cross-trial comparisons (different populations, durations, protocols) - not head-to-head studies.

EndpointSemaglutide 2.4 mgTirzepatide 15 mgRetatrutide 12 mg
Avg weight loss-16.9% (68 wk)-22.5% (72 wk)-28.6% (48 wk)
≥10% responders69.1%86.3%93%
≥20% responders32.0%39.7%63%
Nausea rate44.2%25.9%45%
Discontinuation (AE)7.0%4.3%6.0%
Receptors targetedGLP-1GLP-1, GIPGLP-1, GIP, GCG
CV outcomes dataYes (SELECT)Pending (SURPASS-CVOT)No
FDA statusApproved (2021)Approved (2023)Phase 3
Longest RCT data2 years (STEP 5)72 weeks48 weeks

Cross-trial comparison; populations and protocols differ. Green highlight indicates best-in-class for that endpoint. Retatrutide data is Phase 2 (smaller population).

Beyond Weight

What else the trials revealed

Weight loss is the primary endpoint, but GLP-1 trials have uncovered a striking range of secondary health benefits that go far beyond the scale.

  • Cardiovascular: 20% reduction in heart attacks, strokes, and cardiovascular death (SELECT trial). This benefit was independent of weight loss magnitude.
  • Type 2 diabetes: Up to 2.1% HbA1c reduction. Many participants' diabetes went into remission during treatment.
  • Blood pressure: Average systolic reduction of 4-6 mmHg across STEP and SURMOUNT trials.
  • Sleep apnea: SURMOUNT-OSA trial showed tirzepatide reduced sleep apnea severity (AHI) by ~60%, with some participants no longer meeting diagnostic criteria.
  • Liver fat: Both semaglutide and tirzepatide significantly reduced liver fat content and improved NASH/MASLD markers. Retatrutide's glucagon component may be particularly effective here.
  • Inflammatory markers: CRP (C-reactive protein) reduced by 30-50% across multiple trials, indicating systemic inflammation reduction.
  • Kidney function: The FLOW trial showed semaglutide reduced kidney disease progression by 24% in patients with type 2 diabetes and chronic kidney disease.

The weight regain question

One of the most important findings across all GLP-1 trials: weight regain occurs when treatment stops.

  • STEP 4: 68% of lost weight regained within 1 year of stopping semaglutide
  • SURMOUNT-4: 14% regain vs 2.4% continued loss after tirzepatide withdrawal
  • Interpretation: Obesity is a chronic disease. GLP-1 therapy manages it the way blood pressure medication manages hypertension - ongoing treatment maintains the benefit
  • Emerging approach: Lower maintenance doses after reaching goal weight, reducing cost while preserving most benefits
What's Next

Ongoing & upcoming research

Enrolling

TRIUMPH (Retatrutide Phase 3)

Eli Lilly's Phase 3 program for retatrutide. Multiple trials evaluating efficacy and safety in larger populations (3,000+ participants) for obesity and type 2 diabetes. Expected readout: 2026-2027.

Ongoing

SURPASS-CVOT (Tirzepatide)

Cardiovascular outcomes trial for tirzepatide, similar in design to SELECT. Will determine if tirzepatide provides cardiovascular protection like semaglutide. ~15,000 participants. Expected readout: 2027.

Emerging

Oral Semaglutide (High-Dose)

Novo Nordisk is developing oral semaglutide at higher doses (25 mg and 50 mg) for obesity. The OASIS trial showed 17.4% weight loss with oral 50 mg - comparable to injectable. Could eliminate needles entirely.

Phase 2

Amycretin (Novo Nordisk)

A novel GLP-1/amylin dual agonist. Early Phase 2 data showed approximately 13% weight loss in just 12 weeks. Different mechanism from tirzepatide - amylin further enhances satiety. Watch list compound for 2027-2028.

Phase 2

Survodutide (Boehringer)

Another GLP-1/glucagon dual agonist (not triple). Phase 2 data showed 18.7% weight loss at 46 weeks and impressive NASH/liver fat reduction. Particularly promising for patients with fatty liver disease.

Ongoing

Adolescent & Pediatric Studies

Both semaglutide (STEP TEENS) and tirzepatide are being studied in adolescents aged 12-17. STEP TEENS showed -16.1% weight loss in teens with obesity. Expanding access to younger populations with severe obesity.

FAQ

Research questions

Are these medications safe long-term?
Semaglutide has the most long-term data: the SELECT trial followed 17,604 patients for a median of 3.3 years with no new safety signals. The 2-year STEP 5 trial confirmed sustained efficacy and tolerability. Liraglutide (Saxenda) has been on the market since 2014 with over a decade of real-world safety data. Tirzepatide is newer (2023) but building its safety record. For retatrutide, long-term data will come from the Phase 3 TRIUMPH program. Overall, the GLP-1 class has demonstrated a reassuring long-term safety profile.
Why do different trials show different results?
Trial populations, durations, inclusion criteria, and lifestyle interventions all differ. STEP 1 (semaglutide) enrolled patients with BMI ≥30 over 68 weeks; SURMOUNT-1 (tirzepatide) enrolled similar patients over 72 weeks; the retatrutide Phase 2 trial was only 48 weeks with a smaller population. Patients with type 2 diabetes consistently lose less weight on GLP-1s than non-diabetic patients. Direct comparison requires head-to-head trials with identical protocols.
Is GLP-1 weight loss comparable to bariatric surgery?
Getting closer. Gastric bypass typically produces 25-30% total body weight loss at 1-2 years. Semaglutide achieves ~17%, tirzepatide ~22.5%, and retatrutide ~28.6% - retatrutide is now within range of less-aggressive surgical approaches (gastric sleeve: ~25-30%). However, surgery remains more durable long-term without ongoing treatment. The best candidates for GLP-1 therapy vs surgery is an active area of research and depends on individual factors including BMI, comorbidities, and patient preference.
Do GLP-1 medications work for everyone?
No. Approximately 5-10% of patients are classified as "non-responders" (less than 5% weight loss). Factors that predict better response include: higher baseline BMI, absence of type 2 diabetes, reaching the full dose, and adherence to lifestyle modifications. If one GLP-1 medication doesn't produce adequate results, switching compounds (e.g., from semaglutide to tirzepatide) often works because the mechanisms differ.
What about muscle loss with these medications?
This is a legitimate concern. STEP 1 DEXA data showed approximately 40% of weight lost was lean mass - consistent with all caloric-restriction weight loss, not unique to GLP-1s. The key factors: adequate protein intake (0.7-1g per pound of body weight), resistance training 2-3x per week, and not losing weight too rapidly. Tirzepatide may have a slight advantage in muscle preservation based on SURMOUNT DEXA data. Clinical supervision that monitors body composition and adjusts the plan accordingly is critical.
Explore More

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Semaglutide

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Retatrutide

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The triple-agonist peptide: mechanism, trial results, and future potential.

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