What are the side effects of taking weight loss injections?

Which side effects do most people actually notice?

The GLP-1 medicines licensed in the UK for weight management (tirzepatide (Mounjaro) and semaglutide (Wegovy)) both work partly by slowing the rate at which the stomach empties. That mechanism is effective for reducing appetite, and it is also the reason the gut tends to register the adjustment first. Nausea is the most commonly reported side effect in clinical trials, followed by loose stools or diarrhoea, constipation, burping and acid reflux. Headache, fatigue and dizziness appear less often but are recognised reactions, particularly in the early weeks. If you want a thorough overview before starting, our page on whether there are side effects to weight loss injections walks through what the evidence says in plain terms.

Injection-site reactions (mild redness, itching or a small bump where you inject) are possible too. Rotating your injection site between the abdomen, thigh and upper arm helps reduce these; our guide to taking weight loss injections covers injection technique in more detail.

The important context: these effects are generally mild to moderate in intensity. In the SURMOUNT-1 trial involving thousands of adults, the large majority of participants who experienced GI side effects reported them as manageable, and discontinuation due to side effects was relatively low. Symptoms typically cluster around the start of treatment and each dose step, then ease. The titration approach (beginning at 2.5mg for tirzepatide, for instance, and moving up in stages) exists specifically to give your system time to settle.

Are there any serious side effects to be aware of?

Serious side effects are infrequent but real, and being aware of them is part of using these medicines safely. In January 2026, the MHRA issued a Drug Safety Update on GLP-1 receptor agonists specifically flagging acute pancreatitis as a known risk: the regulator advised patients and clinicians to be alert to severe, persistent abdominal pain that may radiate to the back, with or without vomiting. If you experience that, stop the injection and seek urgent medical attention rather than waiting to see if it passes.

Other symptoms worth taking seriously include signs of an allergic reaction (swelling of the face or throat, difficulty breathing), significant dehydration caused by prolonged vomiting or diarrhoea, and symptoms pointing to gallbladder problems such as upper-right abdominal pain, fever or jaundice. People with type 2 diabetes who are also on certain diabetes medicines should discuss hypoglycaemia risk with their prescriber before starting.

A practical checking habit that takes under a minute: once a week, scan the injection site before you inject, any persistent lump, redness or hardness worth mentioning should be flagged to your prescriber at the next contact. Small observations like this are easy to overlook and easy to report. Our detailed side-effect resource goes into each of these areas with fuller clinical context if you want to read further.

When should you get medical help rather than waiting it out?

Most nausea or loose stools after a dose change do not need a GP visit. The situations that do require prompt attention are different in character: they are more intense, more persistent, or accompanied by other warning signs. Seek medical help (same day or via 999 if severe) if you develop any of the following while taking a weight loss injection:

Severe stomach pain that does not settle and spreads to your back (pancreatitis concern); inability to keep any fluids down for more than 24 hours (dehydration risk); signs of a severe allergic reaction; chest pain or unexplained shortness of breath; yellowing of the skin or eyes; or any thoughts of self-harm or mood changes that feel out of character.

You can also report suspected side effects formally to the MHRA through the Yellow Card scheme at yellowcard.mhra.gov.uk. The scheme is open to patients and carers, not just healthcare professionals, and reports genuinely contribute to how these medicines are monitored post-approval. If you have concerns between prescriber reviews, our aftercare team is available seven days a week.

How do side effects compare between Mounjaro and Wegovy?

Both medicines share the same broad GI side-effect profile because both slow gastric emptying. The headline difference is mechanism: tirzepatide activates two receptors (GIP and GLP-1) while semaglutide activates one. Whether that translates into a meaningfully different day-to-day tolerability experience varies between individuals, and the clinical trials used different designs, so a clean like-for-like comparison is difficult. What the evidence does show is that both carry the same categories of risk, and the same principles (slow titration, staying hydrated, not forcing yourself to eat if nauseated) apply to both. For a closer look at how these reactions present across both medicines, our guide to what the side effects of weight loss injections actually are covers the detail medicine by medicine.

There is one difference worth knowing about regarding contraception. NHS guidance notes that for tirzepatide specifically, women taking the oral contraceptive pill should use an additional non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because slowed gastric emptying may reduce pill absorption. No equivalent advisory applies to semaglutide. If this applies to you, raise it at consultation; our overview of weight loss injections touches on this alongside other practical considerations.

Deciding which medicine is appropriate (and whether either is right for you at all) is a clinical decision. The NHS England guidance on weight management injections outlines the clinical framework that prescribers work within. At nume, a GPhC-registered Independent Prescriber reviews every consultation personally. If you'd like to explore whether treatment might suit you, checking your eligibility starts with a free consultation, and questions about what the long term effects of weight loss injections look like are very much part of the conversation we encourage you to bring.