Semaglutide vs tirzepatide: which works better for weight loss?

Which works better for weight loss?

For average weight loss, tirzepatide. The SURMOUNT-5 trial, the first direct comparison, found roughly six percentage points more weight loss with tirzepatide than semaglutide at 72 weeks. Waist circumference dropped further too: 18.4 cm versus 13.0 cm on average.

Averages hide individuals. Plenty of people on semaglutide out-lose the tirzepatide average, some people respond weakly to either, and nobody can predict your response curve in advance. Individual results vary, which is why doctors judge treatment on your progress, not the trial mean.

Better on average also does not mean better for you. Semaglutide carries evidence tirzepatide does not yet match, which matters for certain patients, as covered below.

What are semaglutide and tirzepatide?

Semaglutide is a GLP-1 receptor agonist: it mimics one gut hormone that reduces appetite, slows stomach emptying and steadies blood sugar. Tirzepatide activates that same receptor plus a second one, GIP. Semaglutide is the molecule in Ozempic, Wegovy and Rybelsus; tirzepatide is the molecule in Mounjaro.

Both are once-weekly injections into the fat just under the skin, and both start at a low dose that steps up gradually over months. Both are Schedule 4 medicines in South Africa: a doctor must assess you and confirm a treatment plan before a licensed pharmacy can dispense either.

Our guide to how semaglutide works covers the single-hormone mechanism in depth, and what tirzepatide is unpacks the dual action.

What did the SURMOUNT-5 head-to-head trial show?

SURMOUNT-5, published in the New England Journal of Medicine in May 2025, randomised 751 adults with obesity but without diabetes to the highest tolerated dose of tirzepatide or semaglutide for 72 weeks. Tirzepatide averaged 20.2% weight loss versus 13.7% for semaglutide. Individual results vary.

The result matched what the separate trial programmes had hinted at for years. Semaglutide's STEP 1 trial produced 14.9% average loss over 68 weeks; tirzepatide's SURMOUNT-1 trial produced around 21% at the top dose over 72 weeks. Comparing across different trials is unreliable, though, which is why a direct randomised comparison was needed before anyone could say it with confidence.

The gap widened at the extremes: 19.7% of the tirzepatide group lost 30% or more of their body weight, against 6.9% on semaglutide. Both groups paired medication with diet and activity support, which is the only setting in which these figures apply.

Semaglutide's 13.7% is still a large, clinically important result: before this generation of medicines, a weight-loss drug producing that kind of average loss did not exist. This is a contest between two strong options, not a good one against a weak one.

One design caveat: the trial was open-label, meaning participants knew which medicine they were taking, which can nudge behaviour. The size of the difference makes the overall conclusion solid, but precise numbers deserve that grain of salt.

Why does tirzepatide produce more weight loss on average?

The extra GIP receptor is the leading explanation. GIP is a second incretin hormone released after meals, involved in how the body handles fat and insulin. Activating it alongside GLP-1 appears to deepen the appetite and metabolic effects, though researchers are still mapping exactly how the two hormones interact.

Day to day, the two medicines feel much the same: less hunger, earlier fullness, quieter background thoughts about food. The gap between them shows up in averages measured over months, not in how any single week on treatment feels.

Milligrams are not the reason. The two molecules have different potencies, so tirzepatide's 15 mg maximum is not more medicine than semaglutide's 2.4 mg in any meaningful sense. What differs is the biology each one switches on.

Semaglutide vs tirzepatide at a glance

Individual results vary. Trial percentages are group averages under study conditions, not personal predictions, and the table is a summary of the evidence covered in this article rather than a scorecard.

Which has worse side effects?

Neither, broadly. Both medicines share the same core side-effect profile: nausea, vomiting, diarrhoea, constipation and stomach discomfort, concentrated in the early months and around dose increases. In SURMOUNT-5, gastrointestinal complaints were the most common issue in both groups and mostly mild to moderate, with similar dropout rates.

The serious risks match too. Pancreatitis is rare but possible with either: severe, persistent stomach pain means seek medical care immediately. Gallbladder problems occur more often with rapid weight loss on either medicine.

The exclusion list is shared as well: pregnancy or breastfeeding, a personal or family history of medullary thyroid cancer or MEN2, and caution after previous pancreatitis. A doctor screens for these before confirming any treatment plan.

Management is identical for both: smaller meals, slower eating and steady fluids around dose increases, with a doctor review if symptoms drag on instead of settling. Practical steps are in our guide to managing GLP-1 side effects.

Which is better if you have heart disease?

Semaglutide, on current published evidence. The SELECT trial found 20% fewer major cardiovascular events, meaning heart attack, stroke or death from cardiovascular causes, in adults with overweight or obesity and existing heart disease on semaglutide 2.4 mg. Tirzepatide has no published equivalent in obesity without diabetes yet.

Absence of data is not proof of absence of benefit; it means the question has not been answered for tirzepatide in that population. Doctors treat the two differently on this point because treatment decisions rest on evidence that exists, not evidence that may arrive later.

For someone with a heart history, a proven reduction in cardiovascular events can outweigh a larger average weight loss. For someone without one, the SELECT result carries less weight in the decision. Where the balance lands is a doctor's call, made on your history rather than a general rule.

How do doctors choose between them?

On a short list of concrete factors: heart history, how central maximum weight reduction is to the clinical goal, tolerability, what a pharmacy can reliably stock, and monthly cost. Trial averages set the starting expectation; your own response over the first months then outranks them.

Where maximum weight reduction is the clinical priority, the SURMOUNT-5 result gives tirzepatide the edge on average. Where cardiovascular protection matters most, semaglutide's SELECT evidence pulls the other way. Neither factor decides on its own.

Personal factors matter as much as the trial ranking. Someone who tolerates semaglutide well, is already losing steadily and has cardiovascular risk has little reason to change. Starting from scratch with maximum weight loss as the goal points the other way. There is no single winner: the right option is decided by a doctor on assessment, and what to expect from treatment covers how that assessment and the follow-up around it work.

Can you switch from semaglutide to tirzepatide?

Yes, in either direction, as a doctor-managed change. The doses do not convert directly, so the doctor starts the new medicine at a low or middle dose rather than the top, and never runs both at once. Common reasons are a plateau, side effects that will not settle, or persistent stock problems. The change does not restart treatment from zero: the doctor carries what was learned about your response and tolerance into the new plan.

The change itself is straightforward when planned: a review of how you responded, a chosen starting dose, then follow-up while the new medicine beds in. The step-up rules of the new molecule apply from its own schedule, not from where you were on the old one. The tirzepatide ladder is set out in our Mounjaro dosage guide.

Switching is not an upgrade to chase. Moving between two working medicines resets the adjustment period, side effects included, and someone progressing well on semaglutide gains nothing by changing. Do not stop or swap on your own; timing and starting dose need a doctor's managing.

Can you get both in South Africa?

Yes. Tirzepatide arrived first, with Mounjaro launching locally in December 2024, and semaglutide for weight management followed when Wegovy launched in August 2025. Both are dispensed by licensed pharmacies, including Dischem and Clicks, after a doctor's assessment and authorisation.

The registrations matter more than the launch dates. In October 2025 SAHPRA added chronic weight management to Mounjaro's registration, for adults with a BMI of 30 or more, or 27 or more with a weight-related condition, alongside diet and activity changes. Semaglutide's weight-management registration belongs to Wegovy; Ozempic remains a diabetes medicine on paper, a distinction unpacked in Mounjaro vs Ozempic.

Both come as multi-dose pens holding four weekly doses, and both stay in the fridge at 2 to 8 degrees until first use. Individual strengths go in and out of stock at branch level, so whether your pharmacy can hold your dose month after month is a real factor in the choice, not an afterthought.

Which costs less in South Africa?

Semaglutide currently publishes the lower figures at maintenance doses, and competition between the two manufacturers has cut published prices repeatedly through 2025 and 2026. The numbers move often enough that any figure needs a date attached, which is why the ones below carry theirs.

One piece of context first: South Africa regulates private-sector medicine prices through the Single Exit Price system, so the medicine itself costs the same at every pharmacy and small till differences come from the regulated dispensing fee. Nobody lawfully sells either pen meaningfully cheaper than anyone else.

Reuters reported in March 2026 that Novo Nordisk had cut Wegovy's South African price for the second time since launch, taking the lowest dose from R3,090 to R1,873 a month and the top dose down 27% to R3,746. Retail price lists published in 2026 put the Mounjaro KwikPen at roughly R3,600 to R8,200 depending on strength, with each pen holding four weekly doses. We do not sell medicine. Prices vary by pharmacy and dose; confirm with your pharmacy.

A clean like-for-like number does not exist: the two molecules are dosed differently, and the figures shift with each price adjustment. Medical aid rarely changes the sums either, because obesity is not a Prescribed Minimum Benefit condition and most schemes exclude weight-management medicine. The full budgeting picture, consultations and blood tests included, is in GLP-1 treatment costs in South Africa.

Will generic versions change the comparison?

For semaglutide, competition is already in sight: SAHPRA approved South Africa's first generic semaglutide, from Sun Pharma, in July 2026, registered for type 2 diabetes, and the molecule's main patent expires in 2027. Tirzepatide has no registered generic, and its patents run for years yet.

More semaglutide entrants after 2027 would add downward pressure across the class, though a diabetes-registered generic does not automatically change what the weight-management brands charge. If the gap between the two molecules widens on the semaglutide side, cost will pull harder in the comparison than it does today.

None of that moves the clinical picture. A registered generic contains the same molecule, so the trial evidence above applies to the molecule, not the box it arrives in.

What about compounded semaglutide or tirzepatide?

Avoid both. Compounded or copycat semaglutide and tirzepatide sold online, through salons or on social media are not SAHPRA-registered, their contents are not verified, and SAHPRA has warned the public about falsified and unregistered versions repeatedly. No licensed pharmacy dispenses either molecule without a doctor's authorisation.

Every figure in this article comes from trials of the registered medicines. An unregistered vial has no trial behind it, whatever the label claims, so the comparison you are reading does not apply to it. A price far below the pharmacy's is a warning sign, not a bargain.

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