Mounjaro®
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Start journey Learn moreHow the side-effect profiles of Wegovy and Mounjaro compare, and why the decision is personal
The decision most people are actually facing
A lot of people arrive at this comparison holding a misconception: that one of these medicines is the 'gentle' option and one is the 'strong but rough' one. That's not quite how it works. Both act on GLP-1 receptors in the gut and the brain, and both slow gastric emptying, which is precisely why both cause nausea, particularly at the start of treatment or after a dose increase. Mounjaro also activates GIP receptors, making it the only dual-agonist weight-loss medicine licensed in the UK, but that second mechanism doesn't translate into a cleanly different side-effect experience.
The more useful frame is this: which medicine's specific pattern of effects, frequency and timing fits better with your health history, your lifestyle and any other medicines you take? That's a clinical judgement, and it's the one our prescribers work through with you. If you're weighing up Wegovy and Mounjaro more broadly, the side-effect picture is one piece of a larger decision that also covers results evidence, dosing schedules and cost.
What the trials and regulators report for each medicine
The table below draws on the published trial programmes and the UK Summary of Product Characteristics for each medicine. Numbers are averages from placebo-controlled trials; your own experience may differ.
| Side effect / feature | Wegovy (semaglutide 2.4mg) | Mounjaro (tirzepatide up to 15mg) |
|---|---|---|
| Nausea (any grade) | Reported in ~44% of participants in STEP 1 (NEJM, 2021) | Reported in ~30–40% across SURMOUNT-1 dose groups (NEJM, 2022) |
| Diarrhoea | Common; among the most frequently reported GI effects | Common; reported at similar or slightly higher rates at the highest doses |
| Constipation | Reported; less common than nausea | Reported; appears at least as frequently as with semaglutide at higher doses |
| Vomiting | Reported in a meaningful proportion; typically decreases over weeks | Reported; similar pattern of settling with time |
| Injection-site reactions | Mild; pre-filled pen, once weekly | Mild; KwikPen, once weekly; rotate sites |
| Pancreatitis (rare, serious) | Known risk; MHRA Drug Safety Update, January 2026, applies to all GLP-1 medicines | Same warning applies; seek urgent help for severe stomach pain reaching the back |
The MHRA's January 2026 Drug Safety Update is relevant to both medicines: severe, persistent stomach pain that radiates toward the back, with or without vomiting, needs prompt medical attention and should not be dismissed as ordinary nausea. Both medicines carry a Black Triangle (▼) designation, meaning they are subject to additional monitoring by the MHRA.
Where the two medicines genuinely differ
For most people, GI symptoms are the main story with both treatments. A few differences are worth knowing.
Tirzepatide's dual mechanism may produce a slightly different nausea character for some people, though head-to-head evidence on subjective tolerability is limited. What the SURMOUNT-5 trial (published in the New England Journal of Medicine, 2025) did confirm is that tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks, but that trial wasn't designed to compare side-effect burden directly. If you want a closer look at how the two medicines' side effects stack up against each other, our detailed breakdown of Wegovy vs Mounjaro side effects covers what the evidence shows on that specific question.
There is one clinically important difference for women on oral contraceptives. Because tirzepatide slows gastric emptying more markedly in the early weeks, the NHS England guidance advises adding a non-oral contraceptive method for the first four weeks of Mounjaro treatment and for four weeks after each dose increase. The same precaution does not currently apply to semaglutide. If you take the pill, this is worth raising at consultation, and our prescribers ask about it as a matter of course.
People considering a switch, or curious about how the tolerability question tends to play out, can read more about whether Wegovy tends to have fewer side effects than Mounjaro, including what the clinical picture looks like for people who find one medicine easier to tolerate than the other.
What to do if side effects become a problem, and how we support you
Both medicines are started at a low dose for exactly this reason: 2.5mg for tirzepatide, 0.25mg for semaglutide. The first pen's job is to let your system adjust, not to drive significant weight loss. Most GI effects peak in the first couple of weeks after a dose change and then ease. Staying hydrated, eating smaller portions and avoiding high-fat meals all help.
If symptoms stay severe or you're concerned, the right move is to contact your prescriber rather than stopping treatment abruptly or adjusting your dose independently. At nume, aftercare is available seven days a week for exactly this kind of question. If you're still unsure which treatment is the better fit, our guide to what has less side effects, Wegovy or Mounjaro, walks through what the evidence suggests for each. You can also explore how both medicines' side effects are typically managed.
For anything urgent, always contact your GP or call 111. The Yellow Card scheme at yellowcard.mhra.gov.uk also lets you report any suspected side effect directly to the MHRA.
Which medicine suits you is a clinical decision our prescribers make with you, weighing your full health history, your other medicines and what matters most to you. Speak to our prescribers through a free consultation, reviewed the same day.
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