GLP-1 medications for weight loss: what the evidence shows

GLP-1 receptor agonists are a class of medications originally developed for type 2 diabetes that mimic glucagon-like peptide-1, a hormone released by the gut after eating. Semaglutide (sold for weight management as Wegovy) and tirzepatide (sold as Zepbound) act on this pathway to slow gastric emptying and signal to appetite-regulating centers in the brain, reducing hunger and increasing feelings of fullness. Tirzepatide additionally activates receptors for a second gut hormone, GIP (glucose-dependent insulinotropic polypeptide), a dual mechanism that appears to contribute to its larger effect size in trials.^[6]

Both drugs are peptide-based medicines, which is sometimes confused with the unregulated “research peptides” discussed elsewhere on this site. The similarity ends at the chemistry. Semaglutide and tirzepatide are FDA-approved prescription drugs, manufactured to pharmaceutical standards and studied in tens of thousands of trial participants, a different category from compounds sold as “research use only.” See our overview of the peptides marketed for healing for that distinction in more detail.

The trial that changed the conversation was STEP 1, a 68-week randomized, placebo-controlled trial of once-weekly semaglutide 2.4mg in adults with overweight or obesity who did not have diabetes. Participants in both the semaglutide and placebo groups received the same lifestyle counseling throughout. At week 68, the semaglutide group had an average body-weight reduction of 14.9%, compared with 2.4% in the placebo group.^[1]

SURMOUNT-1, a similarly designed 72-week trial, tested tirzepatide at three doses (5mg, 10mg, and 15mg weekly) in adults with obesity or overweight. Average body-weight reductions were 15.0%, 19.5%, and 20.9% at the three doses respectively, compared with 3.1% with placebo.^[2] These are the largest average effect sizes reported for any medication in this drug class to date.

These are trial averages, not guarantees. Individual results varied considerably within both trials, and the populations studied (adults with obesity, or overweight plus a weight-related condition, largely without diabetes) do not represent everyone who might consider these medications.

It is easy to read headline weight-loss percentages and assume the medication does all the work. That is not how either drug was studied or approved. The FDA approved semaglutide (Wegovy) in 2021 as an adjunct to a reduced-calorie diet and increased physical activity for chronic weight management, the first new medication approved for that purpose since 2014.^[4] Zepbound (tirzepatide) was approved on the same basis in November 2023.^[5][6] In both pivotal trials, every participant, drug or placebo, received the same lifestyle counseling; the medication was tested as an addition to that foundation, not a substitute for it.

The part of this story that gets far less attention is what happens when treatment ends. In the STEP 1 trial extension, researchers followed a subset of participants for a further year after they stopped semaglutide at week 68. Those who discontinued the medication regained a substantial share of the weight they had lost, and the cardiometabolic improvements seen while on treatment (blood pressure, lipids, and glucose-related measures) largely reverted toward their pre-treatment levels as well.^[3]

This is one extension study following a subset of one trial’s participants, not a body of evidence as deep as the placebo-controlled results above, but it is consistent with how obesity is understood as a chronic, relapsing condition rather than one that is cured by a fixed course of treatment. It is also why these medications are generally framed, and prescribed, as long-term treatments rather than short-term interventions.

Gastrointestinal symptoms, nausea most often, along with diarrhea, vomiting, and constipation, were the most commonly reported side effects in both STEP 1^[1] and SURMOUNT-1.^[2] They were generally mild to moderate in severity and most pronounced while the dose was being increased, which is why both medications are started at a low dose and escalated gradually over several weeks. A minority of participants in each trial discontinued treatment specifically because of these side effects.

Both medications carry an FDA boxed warning, the agency’s strongest label warning, based on thyroid C-cell tumors observed in rodent studies; whether this translates to human risk has not been established. Because of this, both are contraindicated for anyone with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2.^[4][5] Both labels also direct clinicians to stop the medication if pancreatitis is suspected, and neither is recommended during pregnancy.^[4][5]

Two practical realities shape how these medications play out for most people, and neither shows up in a trial’s efficacy results. Brand-name GLP-1 medications are expensive in markets like the United States, insurance coverage for weight-management indications is inconsistent, and supply has been constrained at various points since approval. Because the trial and real-world evidence both point toward weight regain after stopping, a realistic plan has to account for the medication as an ongoing cost and commitment, not a short course with a fixed endpoint.

None of this changes what the trials found. It does mean the honest starting point for anyone considering treatment is a conversation with a prescribing clinician about access, cost, and what happens if the medication has to be paused or stopped, before starting rather than after.

Most people taking these medications do not experience serious complications, but a smaller set of symptoms warrant prompt medical assessment rather than waiting for a routine follow-up.

Outside of these patterns, the picture above holds: these are effective, generally well-tolerated medications for the right person, and the decision to start, continue, or stop belongs in a conversation with a clinician who knows your health history.

The pivotal trials behind semaglutide and tirzepatide represent a genuine step change in what a medication can do for weight loss, with average reductions the diet and exercise literature rarely shows on its own. That result comes with real conditions attached: both drugs are approved as an adjunct to lifestyle changes rather than a substitute for them, the benefit fades after stopping according to the extension data available so far, side effects are common even if usually manageable, and specific personal and family histories rule the medications out for some people entirely. See how we grade the strength of evidence across topics in our editorial and evidence standards, and browse the rest of our Weight & Metabolic Health coverage as it grows.

None of this replaces an individual conversation with a prescribing clinician who knows your health history, weight-related conditions, and goals. It reflects what the pivotal trials and FDA labeling actually show, which is a genuinely effective tool with real trade-offs, not a cure and not a shortcut.