GLP-1 class: semaglutide, tirzepatide, retatrutide compared

The GLP-1 conversation in 2026 is really three conversations. Semaglutide activates one receptor. Tirzepatide activates two. Retatrutide activates three. Each step up the receptor count comes with larger weight-loss effects and a different safety trade-off. Understanding the class means understanding those receptors.

The incretin system, in one paragraph#

The human gut secretes two primary incretin hormones after meals: GLP-1 and GIP. Both stimulate pancreatic insulin release in response to glucose, which is why they're called incretins (INcretin = INtestinal seCRETion of INsulin). A third hormone, glucagon, is secreted by pancreatic alpha cells and opposes insulin, it raises blood glucose and mobilizes stored energy. In healthy metabolism, these three signals are tightly coordinated. In obesity and type-2 diabetes, the coordination degrades. The GLP-1 class of drugs is engineered to restore or amplify incretin signaling pharmacologically.

What GLP-1 activation does (and what semaglutide does)#

Native GLP-1 has a half-life of under two minutes. It's broken down almost immediately by dipeptidyl peptidase-4 (DPP-4). Semaglutide is engineered with modifications that resist DPP-4 degradation, extending the half-life to about one week. The biological effect at each receptor is GLP-1's natural effect, just sustained:

• Appetite suppression: GLP-1 receptors in the hypothalamus and brainstem signal satiety. Patients on semaglutide routinely report feeling full on smaller meals.
• Slowed gastric emptying: Food sits in the stomach longer, which extends satiety and blunts post-meal glucose spikes.
• Glucose-dependent insulin release: Insulin secretion is stimulated when glucose is high and left alone when glucose is normal. This is why GLP-1 agonists carry lower hypoglycemia risk than sulfonylureas.

In the STEP 1 obesity trial, semaglutide 2.4 mg weekly has been shown to produce 14.9% mean body-weight reduction at 68 weeks. FDA-approved as Wegovy for obesity and Ozempic for type-2 diabetes. Approved by the EMA and DMP in Norway under the same brand names. Studies have shown the cardiovascular benefit extends beyond glycemic control alone, particularly in adults with established cardiovascular disease.

What GIP adds (and what tirzepatide does)#

GIP is the other incretin. Its biology is subtler than GLP-1. In addition to augmenting insulin release, GIP has effects on adipose tissue and appears to modulate energy balance through mechanisms that are still being characterized. In isolation, GIP agonism is a weak weight-loss signal. In combination with GLP-1 agonism, the pair outperforms either alone, an additive effect that clinical trials have confirmed rather than predicted.

Tirzepatide is engineered as a single 39-amino-acid peptide that activates both GIP and GLP-1 receptors. Like semaglutide, it's modified for a long half-life and weekly dosing.

In SURMOUNT-1, tirzepatide 15 mg weekly produced approximately 20.9% mean body-weight reduction (intention-to-treat) at 72 weeks in adults with obesity, roughly 40% more than semaglutide's 14.9% in STEP 1 on the same estimand. A head-to-head NEJM trial in 2025 directly compared the two drugs and confirmed tirzepatide's advantage: -20.2% with tirzepatide vs -13.7% with semaglutide at 72 weeks.

The side-effect profile is similar to semaglutide: gastrointestinal events (nausea, vomiting, diarrhea, constipation) dominate, particularly during dose escalation. Tirzepatide is FDA-approved as Zepbound for obesity and Mounjaro for type-2 diabetes. EMA-approved under the same names. Not yet reimbursed for obesity indication under Norwegian health coverage.

What glucagon agonism adds (and what retatrutide does)#

Adding glucagon to the mix is counter-intuitive. Glucagon raises blood glucose, the opposite of what obesity pharmacotherapy aims at. The rationale is that in a carefully balanced triple agonist, glucagon's other effects come forward: increased resting energy expenditure, hepatic fat mobilization, and lipolysis. The GLP-1 and GIP components manage the glucose side; glucagon takes the weight-loss ceiling higher.

Retatrutide is the first molecule to engineer this triple activation into a single 39-amino-acid peptide. In Phase 3 TRIUMPH-4 (December 2025), the 12 mg dose produced 23.7% mean body-weight reduction at 68 weeks, with 58.6% of participants losing at least 25% of body weight, the largest result ever reported in an obesity Phase 3 trial.

The price of that efficacy is a heavier tolerability profile. In TRIUMPH-4, dysesthesia (altered skin sensation) occurred in 20.9% of patients at the 12 mg dose versus 0.7% on placebo. That signal is not specific to glucagon agonism: semaglutide 7.2 mg produced dysesthesia in 22% of patients in its own trials. Discontinuation because of adverse events was 18.2% at 12 mg, well above the 4% on placebo, and Lilly reported the rate was strongly linked to baseline BMI (12.1% at 12 mg among participants with a BMI of 35 or more).

Retatrutide is not approved by the FDA, the EMA or any other regulator as of October 2026. Further Phase 3 trials (TRIUMPH-1, -2 and -3, and TRANSCEND-T2D-1) reported during 2026, and Lilly plans to submit it to the FDA in the first quarter of 2027. For the full breakdown, see the retatrutide guide.

Side-by-side: the numbers that matter#

All three weight-loss figures are intention-to-treat (treatment-policy or treatment-regimen) estimands, so the row compares like with like. The pattern: adding GIP was worth about 6 percentage points over semaglutide 2.4 mg, and adding glucagon about 3 more, though TRIUMPH-4 enrolled a different, heavier population. Semaglutide and tirzepatide share a gastrointestinal profile; retatrutide adds more dysesthesia and a modest heart-rate increase.

Beyond weight loss: T2D, MASLD, and cardiovascular outcomes#

The GLP-1 class is not only about weight. Each molecule has an expanding evidence base in adjacent indications:

• Type-2 diabetes: All three have strong efficacy on HbA1c reduction. Semaglutide and tirzepatide are approved; retatrutide's Phase 3 TRANSCEND-T2D-1 reported out in March 2026.
• MASLD and MASH (metabolic dysfunction-associated liver disease): Semaglutide (Wegovy) received FDA accelerated approval for noncirrhotic MASH with moderate to advanced fibrosis in August 2025, based on the Phase 3 ESSENCE trial. Retatrutide has shown large liver-fat reductions in earlier trials.
• Cardiovascular outcomes: Semaglutide's SELECT trial established cardiovascular benefit in adults with overweight or obesity and cardiovascular disease, without diabetes. Tirzepatide's SURPASS-CVOT, in type 2 diabetes, was noninferior to dulaglutide; its placebo-controlled trial in obesity is still running. Retatrutide's dedicated cardiovascular outcomes trial is still ongoing.

The class is maturing into broadly metabolic therapy, not a narrow weight-loss class.

Mechanism and effect guides: How Does Zepbound Work, and How Long Until It Does?, Ozempic Face: What It Is, and Why It Is Not Ozempic.

Practical takeaways for protocol design#

Three heuristics emerge from the class data:

1. Receptor count correlates with efficacy and with side-effect burden. More receptors engaged means more weight loss and more tolerability risk. The right choice depends on the individual's goal, their tolerance for side effects, and their baseline metabolic profile.
2. Titration is class-wide. Every approved GLP-1 has a mandatory dose-escalation schedule. Starting at target dose is the fastest path to discontinuation. The Klarovel titration calculator builds the week-by-week schedule for any approved GLP-1.
3. Screen first. The contraindication screen (thyroid cancer history, pancreatitis, gastroparesis, pregnancy plans) comes before anything else, and a prescriber will typically check HbA1c, a metabolic panel and lipids. Klarovel's questionnaire builds the protocol from your health profile and screens for these contraindications; blood work is optional and sharpens it if you have it.

For the individual drug breakdowns, see: semaglutide guide, tirzepatide guide, retatrutide guide, and the semaglutide vs tirzepatide head-to-head comparison.

The GLP-1 class is the most important pharmacology story in metabolic medicine right now. Understanding the receptor differences is the difference between reading labels and understanding what you're reading. For how this class fits alongside the rest of the field, see the broader guide to peptides for weight loss.