The number on the scale is the easiest thing to measure and the least interesting result in the GLP-1 literature. Over the past five years the manufacturers ran a series of dedicated outcome trials asking a harder question: does this class of drug change what actually happens to people? Fewer heart attacks. Slower kidney decline. Less pain climbing stairs. Fewer interruptions to breathing at night.
Most of those trials reported positive results. One notable set did not. This article walks through what each study measured, in which population, and what the finding does and does not license you to conclude. Every figure below comes from a named, published phase 3 trial, and where the evidence is preliminary or negative, it is labelled as such.
Why the benefits are not just a side effect of weight loss
GLP-1 receptors are not confined to the pancreas and the appetite centres of the brain. They appear in blood vessel walls, the kidney, the heart, and immune cells. That distribution is the mechanistic reason to expect effects that do not simply follow from a smaller body.
SELECT is the clearest evidence for this. The 20% reduction in major adverse cardiovascular events emerged earlier than the weight loss curve would predict, and it held across baseline BMI categories, including participants at the lower end of the overweight range. Statistical analyses found that the amount of weight lost explained only part of the cardiovascular benefit. Reductions in systolic blood pressure, waist circumference, triglycerides and C-reactive protein, an inflammation marker, all moved in parallel.
This matters practically. If the entire benefit were downstream of weight loss, any method of losing the same weight would produce the same result, and the drug would be a convenience. The evidence suggests otherwise, at least for cardiovascular outcomes.
Cardiovascular disease: the strongest evidence in the class
SELECT (NEJM, 2023) enrolled over 17,000 adults with established cardiovascular disease and overweight or obesity, but without diabetes, and followed them for a median of more than three years. Semaglutide 2.4 mg reduced the composite of cardiovascular death, non-fatal heart attack and non-fatal stroke by 20% compared with placebo. All three components of that endpoint contributed to the result.
This was the trial that changed how the drug is discussed. It moved semaglutide from a weight-loss product to a cardiovascular medication that also causes weight loss, and it is the basis for the expanded labelling in patients with heart disease.
STEP-HFpEF (NEJM, 2023) looked at a group with few good options: people with heart failure with preserved ejection fraction and obesity. Semaglutide produced larger improvements than placebo in heart-failure symptoms, physical limitation and six-minute walk distance, alongside reductions in weight and C-reactive protein. A companion trial, STEP-HFpEF DM, extended the finding to patients who also had type 2 diabetes.
A 20% risk reduction is a relative figure. If the baseline risk of an event over three years is 8%, a 20% relative reduction takes it to about 6.4% - a 1.6 percentage point absolute difference. That is a genuinely worthwhile result at population scale, and it is also smaller than the headline number sounds.
The population matters just as much. SELECT enrolled people who already had cardiovascular disease. Their baseline risk was high, which is what makes a relative reduction translate into meaningful absolute benefit. The same relative figure in a low-risk population would prevent far fewer events.
Kidney disease
FLOW (NEJM, 2024) tested semaglutide 1.0 mg in people with type 2 diabetes and chronic kidney disease. The trial was stopped early for efficacy after an interim analysis showed a 24% reduction in the primary composite endpoint, which combined kidney failure, substantial loss of kidney function, and death from kidney or cardiovascular causes.
Stopping a trial early for benefit is uncommon and requires a pre-specified threshold to be crossed. FLOW also showed reductions in cardiovascular death and all-cause mortality in this population, which is a high-risk group where progression to dialysis is the outcome everyone is trying to avoid.
Blood sugar and the metabolic picture
This is the original indication and the most thoroughly documented. Across the SURPASS programme, tirzepatide lowered A1C by roughly 1.9% to 2.6% depending on dose and comparator. Semaglutide typically lands in the 1.5% to 2.0% range. Neither causes hypoglycemia on its own, because the insulin release they stimulate is glucose-dependent: the effect switches off when blood sugar is normal.
For people with prediabetes, the relevant finding from the STEP programme is that a large majority reverted to normal glucose regulation during treatment. That is a durable-sounding result with an important caveat covered further down: it depends on staying on the drug.
Liver disease
ESSENCE (NEJM, 2025) is the phase 3 trial of semaglutide 2.4 mg in metabolic dysfunction-associated steatohepatitis, or MASH, with moderate to advanced fibrosis. At 72 weeks, 36.8% of patients on semaglutide showed improvement in liver fibrosis without worsening steatohepatitis, and the trial met both of its primary histological endpoints.
Fibrosis is the feature of fatty liver disease that predicts hard outcomes such as cirrhosis. A drug that reverses it in a meaningful fraction of patients addresses a condition that, until recently, had essentially no approved pharmacological treatment.
Obstructive sleep apnea
SURMOUNT-OSA (NEJM, 2024) enrolled adults with moderate-to-severe obstructive sleep apnea and obesity, both those using CPAP and those who were not. Tirzepatide reduced the apnea-hypopnea index, the count of breathing interruptions per hour of sleep, by up to roughly 63% - about 30 fewer events per hour. Hypoxic burden and C-reactive protein fell as well.
For a condition usually managed with a machine rather than a medication, that is a substantive alternative or adjunct, and it is the basis for tirzepatide's approval in this indication.
Joint pain
STEP 9 (NEJM, 2024) measured knee osteoarthritis pain using the WOMAC scale in people with obesity. At 68 weeks, pain scores fell 41.7 points with semaglutide compared with 27.5 points on placebo. Physical function scores improved in parallel.
Mechanically this is less surprising than the cardiovascular results - less load on the joint means less pain - but it is a quality-of-life outcome that patients feel daily, and it was large enough to separate clearly from a placebo group that also received lifestyle counselling.
Blood pressure and inflammation
An individual patient data meta-analysis found a mean systolic blood pressure reduction of about 5 mmHg on semaglutide. In cardiovascular terms that is a clinically significant shift, though it is not equivalent to antihypertensive therapy and should not prompt anyone to change their blood pressure medication without their prescriber.
Inflammatory and lipid markers move consistently across the STEP and SURMOUNT programmes: C-reactive protein and triglycerides fall, and waist circumference drops proportionally more than total weight in some analyses, which points to preferential loss of visceral fat.
Where the evidence is preliminary
Read this section differently. Everything above rests on completed phase 3 trials with pre-specified endpoints. What follows is early, small, or observational. It is worth knowing about and is not a basis for a treatment decision.
Alcohol use disorder
A randomised trial of low-dose semaglutide in adults with alcohol use disorder found reductions in the amount consumed per drinking occasion and in heavy drinking days over nine weeks, though not every consumption measure moved. Observational cohorts have reported substantially lower rates of alcohol use disorder among people taking semaglutide compared with other anti-obesity medications. The mechanism proposed is the same reward pathway that dampens food cravings. The trials are small and short, and this is not an approved use.
PCOS and fertility
Small studies and the early findings from the RESTORE trial suggest semaglutide may restore ovulatory cycles in women with polycystic ovary syndrome, plausibly through weight loss and improved insulin sensitivity. This sits alongside an important safety point: GLP-1s are not appropriate during pregnancy, and anyone using one while trying to conceive needs a clear plan with their clinician for stopping.
Cognition and dementia: a negative result
Observational data had suggested GLP-1 users developed dementia at lower rates, and the hypothesis was taken seriously enough to fund two large phase 3 trials. EVOKE and EVOKE+ did not succeed. Oral semaglutide failed to slow progression in people with early-stage Alzheimer's disease.
This is worth stating plainly because it is the most common over-claim in the current coverage of this drug class. A negative phase 3 result in symptomatic disease does not settle whether long-term use affects dementia risk in healthy people - that is a different question these trials were not designed to answer - but the treatment claim is not supported.
The trial results at a glance
| Trial | Drug | Population | Key finding |
|---|---|---|---|
| STEP 1NEJM, 2021 | Semaglutide 2.4 mg | Adults with overweight or obesity, without diabetes | -14.9% body weight at 68 weeks vs -2.4% on placebo |
| SURMOUNT-1NEJM, 2022 | Tirzepatide 10-15 mg | Adults with obesity, without diabetes | -19.5% and -20.9% body weight at 72 weeks |
| SELECTNEJM, 2023 | Semaglutide 2.4 mg | Established cardiovascular disease, overweight or obesity, no diabetes | 20% reduction in major adverse cardiovascular events |
| STEP-HFpEFNEJM, 2023 | Semaglutide 2.4 mg | Heart failure with preserved ejection fraction plus obesity | Larger improvements in symptoms, physical limitation and six-minute walk distance |
| FLOWNEJM, 2024 | Semaglutide 1.0 mg | Type 2 diabetes with chronic kidney disease | 24% lower risk of major kidney events; stopped early for efficacy |
| SURMOUNT-OSANEJM, 2024 | Tirzepatide | Moderate-to-severe obstructive sleep apnea plus obesity | Apnea-hypopnea index reduced by up to about 63% |
| STEP 9NEJM, 2024 | Semaglutide 2.4 mg | Knee osteoarthritis plus obesity | WOMAC pain -41.7 points vs -27.5 on placebo at 68 weeks |
| ESSENCENEJM, 2025 | Semaglutide 2.4 mg | MASH with moderate or advanced liver fibrosis | 36.8% had fibrosis improvement without worsening steatohepatitis at 72 weeks |
The costs that come with the benefits
An honest account of this drug class has to hold the trade-offs in view alongside the outcome data.
Lean mass
Estimates of how much of the weight lost is lean mass rather than fat range from roughly 25% to 40%, with the higher end associated with higher doses. Some lean mass loss accompanies any substantial weight loss, including from diet alone, but the pace of loss on a GLP-1 makes it worth managing deliberately. The standard countermeasures are adequate protein, generally 1.2 to 1.6 grams per kilogram of body weight, and consistent resistance training. This is the single most actionable thing most patients are not told at the start.
Gastrointestinal side effects
Nausea, vomiting, diarrhea and constipation are common and are usually worst in the days after a dose increase. They are the leading reason people discontinue. Slower titration usually resolves the problem. Rarer but serious risks include pancreatitis and gallbladder disease. Our guide to managing GLP-1 side effects covers the practical approach in detail.
The benefits are conditional on continuing
This is the finding that reframes everything above. When STEP 1 participants stopped semaglutide, they regained about two-thirds of the weight they had lost within a year. In SURMOUNT-4, participants switched to placebo after a 36-week lead-in regained roughly 14% of body weight over the following year, while those who stayed on tirzepatide continued to lose. Cardiometabolic improvements track the weight, so they recede with it.
The practical implication is that these are treatments for a chronic condition, not a course of therapy with an end date. That reality is what makes the monthly price the most consequential number in the whole decision.
What this means for choosing between semaglutide and tirzepatide
- Dedicated trials in cardiovascular disease, kidney disease, heart failure, liver disease and knee osteoarthritis
- Longer real-world safety record
- Substantially cheaper in compounded form
- Lower average weight loss than tirzepatide
- Highest average weight loss of any approved option, around 20.9% in SURMOUNT-1
- Dedicated sleep apnea trial and indication
- Larger A1C reductions across the SURPASS programme
- Shorter outcomes record and a higher monthly cost
If a specific condition on the list above applies to you, that argues for the drug that was studied in it. If the goal is maximum weight loss and nothing else on the list applies, tirzepatide has the stronger numbers. For most people the practical question is which one they can afford to stay on, which is covered in our full tirzepatide vs semaglutide comparison and our GLP-1 cost guide.
Getting access without paying brand-name prices
Brand-name GLP-1s run roughly $900 to $1,400 per month at retail without insurance. Given that the benefits above depend on staying on treatment, that price is the binding constraint for most people rather than any clinical consideration.
Compounded semaglutide and tirzepatide from FDA-registered 503B outsourcing facilities contain the same active molecules at a fraction of that cost. Of the providers we have reviewed, CoreAge Rx remains our top recommendation: compounded semaglutide starts at $99 per month on the 12-month plan and tirzepatide at $299 per month, with US-licensed prescribers and no membership fee. Our ranking of compounded GLP-1 providers explains the criteria in full.
Every trial described in this article studied the branded product at a specific dose. Compounded versions contain the same active molecule, so the mechanism is the same, but they were not themselves tested in these studies and are not FDA-approved products. That is a real distinction, and it is the reason pharmacy sourcing and independent potency testing are the questions worth asking a provider before you sign up.
Frequently asked questions
Do GLP-1 benefits come only from losing weight?
Not entirely. In SELECT, the 20% reduction in major cardiovascular events appeared early and was only partly explained by the amount of weight participants lost, and the benefit was seen across baseline BMI categories. That points to direct effects on inflammation, blood vessel function and blood pressure alongside the weight loss itself.
Which GLP-1 has the strongest evidence outside of weight loss?
Semaglutide, by volume. It has dedicated outcome trials in cardiovascular disease (SELECT), chronic kidney disease (FLOW), heart failure with preserved ejection fraction (STEP-HFpEF), liver disease (ESSENCE) and knee osteoarthritis (STEP 9). Tirzepatide has the stronger weight loss data and its own sleep apnea trial (SURMOUNT-OSA), but a shorter outcomes record overall.
Can a GLP-1 replace my blood pressure medication?
No. Semaglutide lowers systolic blood pressure by roughly 5 mmHg on average, which is meaningful but is not a substitute for antihypertensive therapy. Any change to blood pressure medication has to be made by your prescriber, who may adjust doses as your weight falls.
Do GLP-1 medications protect against dementia?
The evidence does not support that claim. Observational data suggested a link, but the phase 3 EVOKE and EVOKE+ trials found that oral semaglutide did not slow progression in people with early-stage Alzheimer's disease. Prevention in healthy people remains an open question that these trials were not designed to answer.
How much muscle do you lose on a GLP-1?
Estimates of lean mass as a share of total weight lost range from about 25% to 40%, with the higher figures at higher doses. Some lean mass loss is expected with any substantial weight loss. Adequate protein, generally 1.2 to 1.6 grams per kilogram of body weight, and regular resistance training are the standard countermeasures.
Do the benefits stop if I stop the medication?
Largely, yes. STEP 1 participants regained about two-thirds of their lost weight within a year of stopping semaglutide, and in SURMOUNT-4 those switched to placebo regained roughly 14% of body weight while participants who stayed on tirzepatide lost more. Cardiometabolic improvements track the weight, so they reverse alongside it.
Are these benefits proven for compounded GLP-1s?
The trials studied the FDA-approved branded products at specific doses. Compounded semaglutide and tirzepatide contain the same active molecules, so the same mechanism applies, but compounded formulations were not themselves tested in these trials. Sourcing from an FDA-registered 503B outsourcing facility is the key quality signal.
Do I need a qualifying condition to get these benefits?
Each trial enrolled a specific population, usually adults with obesity plus the condition under study. The results describe what happened in those groups. Whether they extend to someone without the condition is a reasonable hypothesis in some cases and unproven in others, which is why prescribing decisions belong with a clinician.
The bottom line
The case for GLP-1 medications no longer rests on weight loss alone. Across eight major trials the class has reduced cardiovascular events, slowed kidney disease, improved liver histology, cut sleep apnea severity and relieved joint pain, in each case in a defined population with the condition being studied. The evidence is strongest for semaglutide simply because it has been studied in more indications for longer.
Three qualifications carry equal weight. The results describe averages in specific groups, not guarantees for any individual. The class has real costs, including lean mass loss and gastrointestinal side effects. And essentially all of it is conditional on continuing treatment, which makes affordability a clinical variable rather than a financial footnote.
If you want to see how these numbers translate to your own starting weight and goal, the planner on our homepage estimates a timeline from the same trial data used throughout this article.