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Most side effects are mild, temporary, and gone within a few weeks
Nausea is the one everyone talks about. It is usually worst in the first week or two, then settles as your body gets used to the medicine.
Alongside it you might get diarrhoea, constipation, bloating, or a bit of stomach ache. Some people also report a headache or feeling more tired than usual in the early weeks. None of these is pleasant. None of them usually lasts.
One patient decided to skip ahead from 2.5 mg to 7.5 mg of Mounjaro in three weeks because he wanted to speed things up. He spent the next 48 hours on the bathroom floor. Once he went back to the official monthly schedule, the weight came off steadily and the nausea went away.
Most of it is managed with small, bland meals and plenty of water. Hold off on anything greasy or spicy in the first fortnight (yes, that includes the Friday night takeaway). Your clinician will step your dose up slowly, at least four weeks at each level, for exactly this reason.
The side effects are a feature of how the medicine works, not a flaw
These injections mimic a hormone your gut already makes. That hormone, GLP-1, tells your stomach to empty more slowly and your brain that you're full. Mounjaro adds a second hormone, GIP, on top of that.
Slower stomach emptying is what cuts your appetite. It is also what makes you feel queasy for a week or two. In other words, the nausea is the medicine doing its job, not a sign something has gone wrong.
I find this is where people panic. They feel a bit sick and assume the medicine is rejecting them. It isn't. It is settling in.
That is why the dose goes up so slowly. You stay on a lower dose for at least four weeks before stepping up, which gives your stomach time to adapt. Rushing it is what lands people on the bathroom floor, not the medicine itself.
The trade-off is worth it for most people. In trials, Mounjaro led to average weight loss of up to 20.9% of body weight, and Wegovy up to 14.9%. A queasy first week looks different against numbers like those. You can see how the options compare in our breakdown of the best weight loss injections.
A handful of side effects need a doctor, not a Google search
A small number of people get something more serious. Pancreatitis, an inflamed pancreas, and gallbladder problems are the two that matter. Both are rare.
The warning signs are specific. Severe pain in the upper part of your stomach that spreads to your back. Vomiting you cannot keep anything down with. Yellowing of the skin or whites of the eyes. Pain that does not settle.
If any of those show up, stop and get help. Call NHS 111 for advice, or go to A&E and call 999 if it is sudden and severe. Do not write it off as just the dose. Pain that does not settle is never normal.
If you do get a side effect, you can report it through the MHRA Yellow Card scheme. It is how the medicines regulator keeps track of safety in the real world.
The real risk isn't the injection. It's where you buy it
Here is the thing nobody tells you in the scary headlines. The genuine medicines, prescribed properly, are heavily regulated and generally well tolerated. The genuinely dangerous versions are the ones bought from a stranger on social media.
A proper supply chain keeps the pen between 2°C and 8°C from pharmacy to your door. It is prescribed by a GMC-registered clinician and dispensed by a GPhC-registered pharmacy. Cut corners on any of that and you are no longer buying medicine. You are buying a mystery.
I would rather lose a sale than put someone on a medicine that is not right for them. If it is not for you, I will say so. That is the whole point of a proper clinical assessment, and it is why our prices are flat rather than a teaser rate that doubles at checkout.
Who shouldn't take slimming injections at all
These are serious medicines, and they are not for everyone. If you are pregnant or breastfeeding, they are off the table. So are they if you have had pancreatitis or gallbladder disease before.
A personal or family history of a specific type of thyroid cancer, medullary thyroid cancer, or a condition called MEN2, also rules them out.
Beyond that, they are prescribed for adults with a BMI of 30 or above, or 27 and above with a weight-related condition like high blood pressure or type 2 diabetes. If you do not meet those, a good clinic will tell you so rather than push a sale.
I tell patients this straight, because most clinics won't. Anyone who will prescribe without asking about your medical history is not doing you a favour.
