Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the two medicines compare on tolerability, specific risks and the one place they genuinely differ
Step 1: Understand what both medicines share
Start here, because most people comparing tirzepatide and semaglutide assume they're looking at dramatically different safety profiles. They're not. Both are GLP-1 receptor agonists, tirzepatide adds a second mechanism (GIP) but at the side-effect level the lived experience is remarkably similar for most users.
The NHS medicines page for tirzepatide and its semaglutide equivalent both list nausea, vomiting, diarrhoea, constipation, indigestion, bloating and fatigue as the most commonly reported effects. Injection-site reactions (redness, mild swelling) appear with both. Headache and dizziness are documented for both. In most cases these effects are most noticeable in the first few weeks of treatment or shortly after a dose increase, and they tend to settle. That pattern holds regardless of which medicine you're on.
The titration schedule (starting low and stepping up) exists precisely to give your digestive system time to adjust. A prescriber oversees this process; it's not something to accelerate on your own. If you want the fuller picture of how the gastrointestinal effects compare in more granular detail, the tirzepatide vs semaglutide gastrointestinal side effects comparison covers that specifically.
Step 2: Look at where the trial data diverge
Head-to-head evidence now exists. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, compared tirzepatide directly with semaglutide 2.4mg in adults with obesity. The efficacy results favoured tirzepatide, greater average weight reduction over 72 weeks. On tolerability, the GI side-effect rates were comparable between arms, which is clinically meaningful: tirzepatide's additional GIP mechanism does not appear to substantially worsen the digestive burden versus semaglutide at equivalent therapeutic doses.
Across the individual trial programmes, if you're weighing up whether semaglutide has fewer side effects than tirzepatide, some analyses do suggest tirzepatide's rates of nausea were marginally lower than semaglutide's at comparable weight-loss outcomes, though indirect comparisons carry their own limitations and NICE's committee discussion around TA1026 noted this nuance. The honest takeaway: neither medicine has a clearly cleaner tolerability record across the board. Individual response varies considerably.
For a broader look at the two medicines beyond side effects, including how a newer agent fits into the picture, the retatrutide vs tirzepatide vs semaglutide comparison of weight loss efficacy and side effects covers all three side by side. The Wegovy vs Mounjaro comparison covers efficacy, eligibility and access side by side.
Step 3: Note the one side-effect distinction that matters for your routine
There is one area where tirzepatide carries an additional caution that semaglutide does not. Women taking the oral contraceptive pill alongside tirzepatide should use a non-oral backup method (condoms, for example) for the first four weeks of treatment and for four weeks after each dose increase. Tirzepatide slows gastric emptying, which can reduce absorption of the pill. NHS guidance confirms there is no equivalent evidence of this effect with semaglutide. If you store your contraceptive pill in the same kitchen drawer where you keep your pen in the fridge door each week, that's a routine worth reviewing with your prescriber before you start.
HRT users should note that NHS England advises considering transdermal HRT (patches or gels) while on tirzepatide, again because of the absorption question. The tirzepatide vs semaglutide side effects overview explores contraception and HRT interactions in more detail.
The MHRA's January 2026 Drug Safety Update highlighted acute pancreatitis as an infrequent but serious risk with GLP-1 medicines as a class, severe stomach pain radiating to the back, with or without vomiting, needs prompt medical attention with either medicine. Report suspected side effects via the MHRA Yellow Card scheme.
Side-by-side summary
| Side effect / consideration | Tirzepatide (Mounjaro) | Semaglutide (Wegovy) |
|---|---|---|
| Nausea, vomiting, diarrhoea | Common; most notable at start or dose increase | Common; most notable at start or dose increase |
| Constipation, reflux, bloating | Reported; broadly similar rates | Reported; broadly similar rates |
| Injection-site reactions | Yes (subcutaneous weekly injection) | Yes (subcutaneous weekly injection) |
| Oral contraceptive interaction | Add non-oral backup for 4 weeks at start and each dose increase [NHS guidance] | No equivalent evidence of reduced pill absorption |
| Pancreatitis risk (class effect) | Infrequent but serious; seek urgent help for severe back-radiating abdominal pain [MHRA, Jan 2026] | Infrequent but serious; same urgent-help criteria |
| HRT consideration | Transdermal form advised while on tirzepatide [NHS England] | No specific NHS England guidance to switch form |
A note on Ozempic: Ozempic is also semaglutide, but it is licensed in the UK for type 2 diabetes, not weight management. It is not the same product as Wegovy and is not prescribed by nume for weight loss. If you've seen comparisons involving Ozempic, the tirzepatide vs Ozempic side effects page explains the distinction clearly.
Which of these medicines is appropriate for you depends on your medical history, current medicines, BMI and weight-related conditions. That is a clinical judgement our prescribers make with you, not a decision a comparison table can settle. If you'd like to explore your options, start your free consultation and a GPhC-registered prescriber will review your answers the same day.
Looking to start your weight loss journey?
Take a quick eligibility quiz to explore your options and see how we can support you.