You’ve probably heard all three names in the same week: at the school gate, in a group chat, in a headline over breakfast. Mounjaro vs Wegovy vs Ozempic is something I’m asked about almost every week. Usually the person asking has read three articles, two forum threads and a friend’s message, and now feels less sure than when she started (I know this can be confusing!). What most people really want to know is simple: how do they work, how much weight will I lose, and what could go wrong? None of them is automatically the best. The right one depends on you, and on the plan you build around it.
One works on one hunger signal, the other works on two
After you eat, your gut releases hormones that tell your brain you’ve had enough and help your body manage blood sugar. These medicines copy those signals, so you feel full sooner and want less food.
Wegovy and Ozempic are the same medicine, semaglutide. It acts on one of those hormones, GLP-1. Mounjaro is tirzepatide, and it acts on two, GLP-1 and GIP. That second signal is thought to explain part of why people lose more weight on Mounjaro on average.
Mounjaro and Ozempic are weekly injections. Wegovy is a weekly injection too, and it is now also available as a once-daily tablet. If you dread needles, that may be the deciding factor (I know injections aren’t for everyone!). It’s still semaglutide, so the side effects and safety advice are the same. It does come with a routine: you take it first thing on an empty stomach with a small sip of water, then wait at least half an hour before eating, drinking or taking anything else.
One practical point on the names. Ozempic is licensed in the UK for type 2 diabetes, not weight loss, so I would always rather use Wegovy, which is licensed for exactly that. Wegovy and Mounjaro are both licensed for weight management in adults living with obesity, or who are overweight with a weight-related condition such as high blood pressure. The Wegovy injection is also licensed to lower the risk of heart attack and stroke in people with established cardiovascular disease, which Mounjaro isn’t in the UK at the time of writing. None of them is licensed for cosmetic weight loss.
Getting them on the NHS is harder. Tirzepatide is being phased in slowly, so many people who qualify can’t get it yet. Semaglutide for weight loss is offered through specialist weight management services for up to two years, and the Wegovy injection can now be prescribed by GPs for people with established cardiovascular disease who are overweight. Whichever route you take, my article on eating well through menopause covers the nutrition side.
Mounjaro vs Wegovy vs Ozempic isn't a race to the biggest number
So which one works better? The first trial to compare them head to head was published last year in the New England Journal of Medicine. People taking tirzepatide lost an average of 20.2% of their body weight, noticeably more than those on semaglutide (13.7%).
That’s a real difference, but it isn’t the whole story. Everyone in the trial knew which medicine they were taking, and it used the standard Wegovy dose. A higher dose of Wegovy has since been licensed, and that hasn’t been tested against tirzepatide. An average across hundreds of people also can’t tell you what will happen to you.
In my experience, the results that last depend far more on what surrounds the medicine:
- how well you cope with side effects while the dose goes up
- enough protein and regular strength training, to protect muscle as weight falls
- sleep, stress and alcohol
- hormonal change, particularly in perimenopause and menopause
- thyroid function, other medicines and other health conditions
- regular review, so that treatment that isn’t working is changed rather than continued
The side effects, said plainly
Most side effects come from the gut and are linked to the dose: nausea, vomiting, diarrhoea, constipation, indigestion, tummy pain and tiredness. They’re most common in the early weeks and after each dose increase, and they often settle. Going up more slowly can help.
The rarer complications matter more, so please know these:
- Pancreatitis. The MHRA has recently strengthened its warnings after rare severe and fatal cases. Severe, persistent stomach pain that may spread to your back needs urgent medical attention. If pancreatitis is confirmed, the medicine shouldn’t be restarted.
- Sudden sight loss. The MHRA has confirmed a very rare form of optic nerve damage, called NAION, as a side effect of semaglutide, so this applies to both Wegovy and Ozempic. If your sight suddenly gets worse, go to eye casualty or A&E straight away.
- Gallbladder problems, including gallstones, which are more likely with rapid weight loss.
- Dehydration from vomiting or diarrhoea, which can affect the kidneys.
- Low blood sugar when used alongside insulin or some other diabetes medicines.
- Anaesthesia. Food can stay in the stomach longer, so always tell the team before any operation or sedation.
These medicines aren't for everyone, and that's fine
I take particular care if you’ve had pancreatitis, have significant digestive problems or diabetic eye disease, or have a history of an eating disorder. For some people, these medicines simply aren’t the right choice. That can be hard to hear when it feels as if everyone around you is on one (I know this can be hard!). But it doesn’t mean you’ve run out of options. It means your plan is built differently.
Pregnancy needs particular attention. None of these medicines should be used during pregnancy or while trying to conceive, and the MHRA advised effective contraception for anyone taking them who could become pregnant. If you’re planning a baby, stop semaglutide at least two months beforehand, and tirzepatide at least one month. Semaglutide shouldn’t be used while breastfeeding. Tirzepatide’s UK licence was recently updated to say it could be considered during breastfeeding, so that’s a conversation to have with your doctor.
Tirzepatide can also make the contraceptive pill less effective, so use a non-oral method, or add a barrier method such as condoms, for four weeks after starting and after each dose increase. And one point I make to every woman in perimenopause: HRT is not contraception. Pregnancy is still possible, and contraception is usually needed until around 55.
Only ever get these medicines from a registered prescriber after a proper assessment. The MHRA has repeatedly warned about fake pens sold through unregulated websites and social media.
Plan for the day you stop before you start
This is the part people most often leave out. When semaglutide was stopped in a large trial extension, people regained two-thirds of the weight they had lost within a year. That’s hard to hear, but it doesn’t mean the medicine failed. It means a long-term condition needs a long-term plan, so this conversation belongs at your first appointment, not your last.
Here’s one thing to do before that appointment. Write down three answers: what you’ve already tried (a slimming club, a diet app, a previous medicine), which side effect you’d find hardest to live with, and whether pregnancy is possible for you. I promise they change the conversation. Then add what the medicine can’t do for you: steady meals, enough protein, regular movement and good sleep. My article on why strength training is medicine after 40 explains why protecting muscle matters so much during and after weight loss.
The name on the pen matters less than the plan around it
Mounjaro, Wegovy and Ozempic are not a league table with a winner at the top. One works on two hunger signals and often leads to more weight loss, the other works on one and may suit you better, and for some people the right answer is neither. Choose the medicine, or the approach, that fits your body, your history and your life. Don’t start until you know what comes after it.
In one unhurried 45-minute consultation at 10 Harley Street, Dr Katy Kasraie will look at your health history, your goals and every suitable option, including approaches that do not involve medication. No referral needed, and most major insurers are accepted. Click here to book your appointment.