Side Effects of Weight Loss Medication Injections: What to Expect

The first few weeks: your body adjusting to the injection

Picture this. You've done your first injection on a Sunday evening, and by Tuesday morning your appetite is noticeably lower, but so is your enthusiasm for breakfast, because your stomach feels mildly off. That early queasiness is the experience most people describe, and it is not a sign something has gone wrong.

Weight loss injections such as tirzepatide (Mounjaro) and semaglutide (Wegovy) slow gastric emptying, which is part of how they reduce appetite. That same slowing is also why nausea is so common at the start. Nausea, burping, indigestion, loose stools or constipation (sometimes more than one at once) are all reported by people using these injectable medicines, and you can read a full breakdown of what to expect on our page covering the side effects of weight loss injections. The NHS notes that these effects are generally mild to moderate and often improve within a few days to two weeks as the body settles.

A few practical things make a real difference: eating smaller portions, cutting back on fatty or rich food, staying well hydrated, and not lying down straight after eating. Alcohol can worsen nausea, so many people reduce it during the early weeks. If you're unsure whether what you're experiencing is typical, the team at our aftercare support line is available seven days a week.

Headache, fatigue and dizziness are also reported in the early period, often linked to reduced food and calorie intake rather than the medicine itself. They generally pass.

Side effects that need medical attention, know these before you inject

Most side effects from weight loss injections are manageable at home. A small number are not, and it is worth knowing them before you start rather than searching in a panic at midnight.

Acute pancreatitis is rare, but it can be serious. In January 2026, the MHRA issued a Drug Safety Update for GLP-1 medicines specifically highlighting this risk. The symptom to watch for is severe stomach pain that does not go away and may radiate to the back, with or without vomiting. If that happens, stop eating and drinking and go to A&E or call 999, do not wait to see if it passes.

Other symptoms that warrant prompt medical contact: signs of an allergic reaction (swelling of the face, lips or throat, difficulty breathing), gallbladder pain (severe upper-right abdominal pain, sometimes with fever or jaundice), and symptoms of dehydration from prolonged vomiting or diarrhoea, particularly dry mouth, very dark urine and dizziness on standing. If you are taking insulin or other diabetes medicines alongside your injection, low blood sugar is an additional risk to know about.

Low mood or thoughts of self-harm: these are listed as possible effects in the product information for both tirzepatide and semaglutide. Tell your prescriber or GP promptly if your mental health changes after starting treatment. You can find more detail on the full side-effect profile on the NHS tirzepatide page and its semaglutide equivalent.

Injection-site reactions and longer-term considerations

Redness, itching, a small lump or mild bruising where you injected are common and almost always short-lived. Rotating your injection site each week (abdomen, thigh and upper arm are all licensed sites) reduces the chance of a persistent reaction building up under the skin. A guide to correct injection technique covers this in detail, including the importance of letting the pen reach room temperature before use.

Some people notice hair thinning a few months into treatment. This is generally related to rapid weight loss rather than the medicine specifically (a well-established phenomenon called telogen effluvium) and tends to resolve once weight stabilises. Keeping protein intake adequate helps.

For women on the oral contraceptive pill who are using tirzepatide, there is a specific consideration: because the injection slows gastric emptying, pill absorption may be reduced. The NHS advises adding a non-oral method of contraception for the first four weeks of tirzepatide treatment and for four weeks after each dose increase. This does not apply in the same way to semaglutide, but it is worth discussing both with a prescriber. Our frequently asked questions cover the contraception point in more detail.

If you are considering whether injections are right for you or would prefer a non-injection route, the options are explored on our page about non-injection weight loss medicines.

Reporting side effects and working with your prescriber

Any suspected side effect from a weight loss injection can be reported through the MHRA's Yellow Card scheme at yellowcard.mhra.gov.uk. This applies to both expected effects and anything unusual, the data shapes future safety guidance, so it genuinely matters.

A question our prescribers hear most weeks: "Is what I'm feeling normal?" The answer is almost always yes for the first few weeks. But almost always is not always, and that distinction is exactly why these are prescription-only medicines requiring clinical oversight. Picking up the phone or sending a message is always the right call if you're not sure. Before any prescription is issued, our prescribers discuss side effects and your specific health picture as part of the consultation, it is not a box-ticking exercise.

If you're still weighing up whether to start, our overview of weight loss injections covers how they work and what the clinical evidence says, and the injectable weight loss medicine page gives a practical comparison of what's currently licensed in the UK. When you're ready to talk to a prescriber, starting a free consultation is the first step.