Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the two medicines compare on tolerability: the factors that should shape your decision
The side effects that appear in both medicines, and why that is
Nausea is the symptom patients ask about most, and it tops the list for both tirzepatide and semaglutide. Both work partly by slowing the rate at which food leaves the stomach, so it stands to reason that the body needs time to adjust. In the SURMOUNT-1 trial, nausea occurred in roughly 30% of participants taking tirzepatide 15mg; in the STEP 1 trial of semaglutide 2.4mg, the figure was broadly similar. Vomiting, diarrhoea and constipation follow a comparable pattern across both medicines, as documented on the NHS tirzepatide page and the equivalent semaglutide guidance.
These effects tend to be most noticeable in the first weeks of treatment or after a dose increase, and they often settle. Fatigue, headache and injection-site reactions also appear on both lists. One misconception worth setting aside is the idea that tirzepatide is simply a stronger version of Wegovy and therefore automatically harsher to tolerate, the mechanism is genuinely different, and trial data do not consistently show tirzepatide causing more GI trouble overall. For a closer read of the gastrointestinal data specifically, our detailed GI comparison goes deeper.
Both carry a Black Triangle (▼) status with the MHRA, meaning they are subject to additional monitoring. Reporting any suspected side effect through the MHRA Yellow Card scheme helps build the evidence base for everyone using these medicines.
Where the two medicines diverge on side effects
The differences are real, but they are differences of degree and context rather than kind. The SURMOUNT-5 head-to-head trial (published in the New England Journal of Medicine in 2025) compared tirzepatide directly with semaglutide 2.4mg in adults with obesity. Discontinuation rates due to adverse events were broadly low in both arms, and the trial did not identify a dramatically worse tolerability profile for either medicine.
Where the medicines do differ is in their interaction with oral medicines. Because tirzepatide affects gastric emptying more markedly in some people, the MHRA advises that women taking the oral contraceptive pill should use a non-oral method of contraception (such as condoms) for the first four weeks of tirzepatide treatment and for four weeks after each dose increase. Current evidence does not suggest the same concern applies to semaglutide injection to the same extent. NHS England guidance also suggests considering transdermal HRT (patches or gels) during tirzepatide treatment, and this is worth discussing with your GP.
Injection-site reactions are also documented for both, though the pen designs differ; our Wegovy vs Mounjaro side effects page covers the practical injection experience in more detail. Neither medicine is recommended during pregnancy, while breastfeeding, or when trying to conceive, and neither is licensed for anyone under 18.
Reading the comparison table
| Side effect | Tirzepatide (Mounjaro) | Semaglutide injection (Wegovy) |
|---|---|---|
| Nausea | Common (~30% at highest dose, SURMOUNT-1) | Common (~44% at 2.4mg, STEP 1) |
| Vomiting | Common | Common |
| Diarrhoea | Common | Common |
| Constipation | Common | Common |
| Oral contraceptive absorption | Additional contraception advised for 4 weeks at start and after each dose increase (MHRA) | No equivalent MHRA warning at present |
| Pancreatitis (rare) | Known infrequent risk (urgent help for severe back-radiating stomach pain (MHRA DSU, Jan 2026) | Known infrequent risk) same warning applies |
Trial-reported nausea rates vary by dose, population and how symptoms were collected, these figures come from the respective NEJM publications and should be read as indicative, not directly comparable, because the two trials had different designs. A fuller breakdown of the head-to-head data sits on our dedicated comparison page.
What this means for your decision, and how our prescribers approach it
Choosing between these two medicines on the basis of side-effect profiles alone is rarely straightforward. Your existing medications, any history of GI conditions, how you take contraception, and whether you are considering HRT all feed into the picture. A question our prescribers hear regularly is: which one will make me feel less sick? The honest answer is that neither medicine can promise a nausea-free start, and individual responses vary considerably.
Pancreatitis is an infrequent but serious risk with both medicines. The MHRA highlighted this in a Drug Safety Update in January 2026: if you develop severe, persistent abdominal pain that spreads to your back, seek urgent medical attention and stop the medicine. That warning applies equally to tirzepatide and semaglutide, and if you are also weighing up newer options such as retatrutide, our comparison of retatrutide, tirzepatide and semaglutide on efficacy and side effects sets out what the current evidence shows.
If you want to compare the two medicines more broadly (how they work, what weight loss to expect, and what they cost in the UK) our Wegovy vs Mounjaro overview brings that together. For the cost side specifically, our UK price comparison covers what private treatment typically involves. Both Mounjaro and Wegovy are prescription-only medicines, and which is right for you is a clinical decision our prescribers make with you, not something a comparison page can settle on your behalf. Start your free consultation and a GPhC-registered prescriber will review your answers the same day.
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