Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the clinical evidence compares on safety and tolerability
What the trial data says about side effects in each medicine
The honest starting point is the trial record. In SURMOUNT-1, published in the New England Journal of Medicine, tirzepatide's most commonly reported side effects were nausea, diarrhoea, vomiting and constipation, collectively affecting a meaningful minority of participants, most often in the first weeks of treatment or after a dose step up. The STEP 1 trial for semaglutide 2.4mg, also published in the NEJM, reported a very similar gastrointestinal pattern: nausea and diarrhoea were the leading complaints, again concentrated around the start of treatment.
Because the two medicines were tested in separate trials with different populations, a direct statistical comparison of adverse-event rates is not straightforward. The SURMOUNT-5 head-to-head trial (New England Journal of Medicine, 2025) compared tirzepatide against semaglutide 2.4mg directly and found that tirzepatide produced greater average weight reduction over 72 weeks, but the trial was not designed or powered to declare one medicine safer than the other. Both had broadly similar tolerability profiles in that comparison.
The MHRA monitors both medicines actively. A January 2026 Drug Safety Update flagged acute pancreatitis as a known but infrequent risk with GLP-1 medicines as a class, covering semaglutide, and applicable to tirzepatide through its GLP-1 activity too. Severe, persistent stomach pain that spreads to the back is the key warning sign: seek urgent medical help rather than waiting. You can report any suspected side effect via the MHRA Yellow Card scheme.
Where the two medicines genuinely differ in their safety profile
Semaglutide is a GLP-1 receptor agonist. Tirzepatide acts on two receptors (GIP as well as GLP-1) making it the only dual-agonist weight-loss medicine currently licensed in the UK, and if you are wondering whether tirzepatide is the same as semaglutide, that additional mechanism is a key part of why they differ, even where their side-effect lists look similar on paper.
One practical difference matters specifically for women. For tirzepatide, the NHS England weight-management-injections guidance advises that women using oral contraceptives should add a non-oral method (condoms, for example) for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide's effect on gastric emptying can reduce how much of the pill is absorbed. There is currently no equivalent evidence of reduced contraceptive absorption with semaglutide. This is not a reason to avoid tirzepatide; it is a reason to mention your contraception to your prescriber before starting.
For women on HRT, NHS England also suggests considering a transdermal form (patch or gel) rather than oral HRT while taking tirzepatide, again, something to raise with your doctor. If you are comparing the two medicines for yourself, the fuller picture of how they sit alongside other factors worth weighing between semaglutide and tirzepatide is worth reading before making a decision.
Safety at a glance: comparing the two medicines on key criteria
| Factor | Semaglutide (Wegovy) | Tirzepatide (Mounjaro) |
|---|---|---|
| Mechanism | GLP-1 receptor agonist | Dual GIP + GLP-1 receptor agonist |
| Most common side effects | Nausea, diarrhoea, vomiting, constipation | Nausea, diarrhoea, vomiting, constipation |
| Pancreatitis risk | Known infrequent class risk (MHRA DSU, Jan 2026) | Known infrequent class risk (MHRA DSU, Jan 2026) |
| Oral contraceptive absorption | No current evidence of reduced absorption | Add non-oral method for 4 weeks on starting and after each dose increase |
| MHRA Black Triangle (▼) | Yes) enhanced monitoring | Yes, enhanced monitoring |
| Average weight loss in trials | ~15% at 2.4mg over 68 weeks (STEP 1, NEJM) | ~20–21% at 15mg over 72 weeks (SURMOUNT-1, NEJM) |
Sources: STEP 1 trial, NEJM; SURMOUNT-1, NEJM; NICE TA875; NICE TA1026.
Which medicine is right for you, and how that decision gets made
If you've been searching this question, you're probably weighing two medicines that both sound reasonable on paper and trying to work out which one is actually the safer bet for you personally. That feeling is understandable, and the answer is genuinely individual. Medical history, existing medications, how your body responds to gastric-emptying changes, your contraception, whether you've tried one medicine before: all of it matters.
There is no universal answer to whether tirzepatide is safer than semaglutide, or the other way around, and our page looking at which is safer between semaglutide and tirzepatide sets out what clinical evidence does show: that both carry a similar class-level risk profile, that both are under active MHRA monitoring, and that neither is prescribed without individual clinical assessment for good reason. Our page on whether Wegovy or Mounjaro is safer looks at the branded medicines side by side if that framing is more useful.
For people who want to understand the broader differences before speaking to anyone, our comparison of Wegovy and Mounjaro covers efficacy, eligibility, cost context and practical differences. And if you are already on one medicine and wondering about switching, the experiences of people moving between Mounjaro and Wegovy add some useful real-world texture.
Which medicine suits you is a clinical decision our prescribers make with you, not something that can be settled by a comparison table alone. Check your eligibility and start your free consultation; a GPhC-registered prescriber will review your answers the same day.
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