Side Effects of Weight Loss Injections: the NHS Picture, Plainly Explained

The first few weeks: what most people actually experience

Picture this: it is Tuesday morning, your first Mounjaro pen arrived yesterday, and by Thursday you feel distinctly queasy after lunch. That is probably the most common story our prescribers hear, a wave of nausea in the days after a new dose that makes food feel less appealing than usual. Nausea sits at the top of the side-effect list for both tirzepatide and semaglutide, followed by diarrhoea, constipation, bloating, indigestion and, for some people, burping more than usual.

These effects happen because both medicines slow the rate at which the stomach empties. That is partly what reduces appetite, but it can also leave food sitting longer than the gut expects. Smaller, lower-fat meals tend to ease the discomfort considerably. Fatigue, mild headache and dizziness also appear in the trial data, usually in the early weeks. The NHS medicines page for tirzepatide sets out these common effects clearly, as does the equivalent page for semaglutide, and both are worth reading before your first injection.

Injection-site reactions (a patch of redness, firmness or mild bruising where the pen was used) are also listed as common. Rotating between the abdomen, thigh and upper arm with each weekly dose reduces their frequency. For most people these settle quickly. If they persist or worsen, that is worth flagging with your prescriber. If you are wondering where you can get weight loss injections near you, our overview walks through the options available locally and what to expect at each stage. You can also read more about the treatment options on our weight loss injections overview.

When side effects are not normal: recognising the signs that need urgent attention

A question our prescribers hear most weeks is some version of: "How do I know if my stomach pain is just the injection or something more serious?" The honest answer is that location, severity and duration all matter.

In January 2026, the MHRA issued a Drug Safety Update specifically flagging acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. The warning sign to act on is severe, persistent abdominal pain, particularly if it radiates through to the back and is accompanied by vomiting. That combination is different from the ordinary nausea most people experience. If it happens, stop eating and drinking and get medical help promptly. You can report a suspected side effect directly at the MHRA's Yellow Card scheme, which also accepts reports from patients and carers.

Other symptoms that warrant urgent attention include: signs of a serious allergic reaction (swelling of the face, throat or tongue; difficulty breathing; a fast heartbeat alongside a skin rash); symptoms of gallbladder problems such as upper-right abdominal pain, fever or jaundice; and signs of severe dehydration from prolonged vomiting or diarrhoea, dizziness on standing, very dark urine or passing no urine. None of these are common, but none should be waited out at home. If you are unsure whether to seek help, NHS 111 is available around the clock.

If you are weighing up which treatment might suit your health profile, the guide to weight loss injections on the NHS covers how eligibility and clinical review work together.

Managing side effects day-to-day, and what the NHS guidance recommends

NHS England's guidance for people starting weight management injections is practical on this front. Eating slowly, stopping at the first real sign of fullness, choosing lower-fat foods and staying well hydrated all reduce the intensity of GI symptoms. Breaking meals into smaller portions across the day works better than two or three large ones. Alcohol tends to worsen nausea and can increase the risk of low blood sugar in people also taking certain diabetes medicines, so reducing it during the dose-increase phase is sensible.

Dehydration is a genuine risk if vomiting or diarrhoea is prolonged. Sipping water steadily (rather than gulping to compensate) helps. If you cannot keep fluids down for more than 24 hours, that warrants a call to your GP or pharmacist rather than waiting for the next clinic appointment.

Some people worry about weight loss injections being expensive on top of a GP visit; there is useful context on the private versus NHS route at our guide to the cost of weight loss injections. Equally, if you are still deciding whether treatment is right for you, our treatment overview covers both medicines side by side. For a more specific comparison of what is newly available on the NHS, this page on newer options is a useful next step.

Side effects, clinical review and the decision to continue or adjust

Both tirzepatide and semaglutide use a gradual titration schedule precisely because the body needs time to adjust. The starting dose for tirzepatide is 2.5mg; for semaglutide, 0.25mg. Neither is a therapeutic maintenance dose, they exist to reduce the severity of early side effects before the prescriber moves the dose upward, typically in four-week steps. If side effects at a given level are not settling, a prescriber can pause the titration or discuss other strategies. That flexibility is one reason clinical oversight matters throughout treatment, not only at the start.

The NHS England guidance on weight management injections also addresses the interaction between tirzepatide and oral contraceptives: because gastric emptying slows, pill absorption may be reduced during the first four weeks of treatment and for four weeks after each dose increase, and an additional barrier method is recommended. It is one of the details worth going over at consultation, not after the first pen arrives. People considering HRT alongside treatment will find the same NHS page addresses transdermal options.

At nume, a GPhC-registered prescriber personally reviews every consultation, not a decision engine, a real clinician who reads the full picture. Side effects, current medications and medical history all factor in before a prescription is issued. Nothing is auto-renewed; speak to our prescribers about whether these treatments are suitable for you. If you have questions about what a legitimate private clinic should look like, this guide to choosing a provider is a straight read. And if you want to understand more about who our clinical team are, our clinical lead's profile is on the site.