Two weekly injections, two different mechanisms, and a set of practical differences that matter more than most comparisons admit. Here is what actually separates them, and what should guide the choice.
Mounjaro is tirzepatide. Wegovy is semaglutide. Both are weekly injections that reduce appetite and slow stomach emptying, but Mounjaro acts on two gut hormone receptors where Wegovy acts on one.
Beyond the mechanism, the differences that actually change a decision are practical ones: Mounjaro carries specific advice about oral contraception that Wegovy does not, and the two have different guidance on how long to stop before a planned pregnancy. Both are prescription-only, both are started low and increased in steps, and both need monthly review.
We have deliberately not put a "which loses more weight" figure on this page. Trial averages are widely quoted online, usually without the trial design or the population attached, and comparing them across separate studies is not a like-for-like comparison. What matters more is which one you can tolerate and keep taking, and that is not knowable in advance.
| Mounjaro | Wegovy | |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| How it works | Acts on two gut hormone receptors, GLP-1 and GIP, which together reduce appetite and slow stomach emptying. | Acts on one gut hormone receptor, GLP-1, reducing appetite and slowing stomach emptying. |
| How it is taken | One injection a week, on the same day each week. | One injection a week, on the same day each week. |
| Dose changes | Started low and increased in steps, no sooner than every four weeks. | Started low and increased in steps, typically every four weeks over several months. |
| Prescription status | Prescription-only. Requires assessment by a prescriber before supply. | Prescription-only. Requires assessment by a prescriber before supply. |
| Pregnancy and conception | Not to be used in pregnancy. Stop at least one month before a planned pregnancy. | Not to be used in pregnancy. Stop at least two months before a planned pregnancy. |
| Oral contraception | May reduce absorption of the pill, particularly when starting or increasing the dose. An additional method is advised for four weeks. | No equivalent advice on reduced pill absorption. |
| Common side effects | Nausea, constipation, diarrhoea and reflux, usually worst after a dose increase and easing with time. | Nausea, constipation, diarrhoea and reflux, usually worst after a dose increase and easing with time. |
| Tends to suit | People who want the newer dual-receptor option and can accommodate the contraception advice. | People who want the longer-established option with the broadest prescribing history. |
Current pricing for each treatment is on its own page, where it is read from our live records rather than restated here: Mounjaro and Wegovy.
The mechanism is the headline difference. Semaglutide mimics GLP-1, a hormone your gut releases after eating that signals fullness to the brain and slows how quickly the stomach empties. Tirzepatide does the same and also acts on the GIP receptor, a second gut hormone pathway involved in how the body handles food. Whether that dual action matters for you specifically is not something a website can tell you.
The contraception advice is the difference most likely to change a decision in practice. Tirzepatide can reduce absorption of oral contraceptives, particularly when starting treatment and for four weeks after each dose increase, and an additional or alternative method is advised during those windows. There is no equivalent advice for semaglutide. If you rely on the pill, say so at the consultation, because it is a genuine reason to prefer one over the other.
Pre-conception timings also differ. Current guidance is to stop tirzepatide at least one month before trying to conceive, and semaglutide at least two months. If you are planning a pregnancy in the next year, that difference is worth factoring in.
More than the marketing suggests. Both are once-weekly injections given with a pre-filled pen, taken on the same day each week. Both start at a low dose that is increased in steps, and both cause the same cluster of gastrointestinal side effects (nausea, constipation, diarrhoea, reflux) which are usually worst in the days after a dose increase and settle with time.
Both are prescription-only and both require assessment before supply. Both need the same care around other medicines, particularly insulin and other diabetes treatments, where doses may need adjusting. Neither is suitable in pregnancy or while breastfeeding, and neither should be taken alongside another GLP-1 medicine.
And both work considerably better with the parts that are not the injection: protein intake, resistance exercise to protect muscle mass, and a plan for what happens when treatment stops.
Why this is worth discussing in person
Every factor above is specific to you, and none of it is visible to an online questionnaire. A twenty minute consultation at the pharmacy settles it, and it is free. If neither medicine is appropriate, that is a legitimate outcome and you will not have paid for anything.
If one is not working, or the side effects remain intolerable after dose adjustment, switching is a reasonable thing to consider. It is not a thing to do yourself. The doses are not equivalent between the two medicines, and starting the second at the level you reached on the first is how people end up genuinely unwell. Raise it at your monthly review and it will be managed as a proper dose change.
Reviewed by the responsible pharmacist at Chatham Pharmacy
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The comparison below narrows it down. The decision itself depends on your history and your other medicines, which is a twenty minute conversation at 139 New Road in Chatham, a few minutes from the station.
We are based on New Road in Chatham, a short walk from the station and with parking nearby. Walk-ins are welcome when slots are free.
The free consultation exists to answer exactly this question with your history in front of us. If neither medicine is right for you, we will say so.