Is it safe to use weight loss injections like Mounjaro and Wegovy?

What does the clinical trial safety data actually show?

Both medicines have been through extensive phase-3 programmes before reaching UK patients. The SURMOUNT-1 trial, which tested tirzepatide, randomised 2,539 adults and ran for 72 weeks; the STEP 1 trial for semaglutide 2.4mg involved a similarly large cohort over 68 weeks. SURMOUNT-1, published in the New England Journal of Medicine, found tirzepatide's side-effect profile was dominated by gastrointestinal symptoms (nausea, vomiting, diarrhoea and constipation) which were generally mild to moderate and most pronounced at the start of treatment or after a dose increase.

The picture for semaglutide is similar. The NHS medicines page for semaglutide sets out the full side-effect list alongside the situations in which people should not take it, in clear language. Both medicines carry a Black Triangle (▼) designation in the UK, meaning the MHRA actively collects additional safety data post-launch, a sign of thorough regulation, not a reason for alarm.

On injection technique and safe disposal, the basics matter: clean the injection site before use, rotate between your abdomen, thigh and upper arm, and dispose of used pens safely. A dedicated sharps container is the straightforward answer for used pens at home.

The short answer: these medicines have a known, manageable safety profile, not an unknown one. That knowledge comes from the rigorous trial process required before any UK licence is granted.

Who should not use these injections, and what needs a prescriber's judgment?

The weight loss injections currently licensed in the UK are not suitable for everyone. People who are pregnant, breastfeeding, or planning a pregnancy are advised against using either medicine. The same applies to anyone under 18. These are not edge-case restrictions, they are hard limits set in the approved prescribing information.

Beyond those absolute exclusions, certain conditions require careful clinical discussion rather than an automatic yes or no. A personal or family history of medullary thyroid carcinoma, or of Multiple Endocrine Neoplasia type 2, is a contraindication. A history of pancreatitis, or certain gastrointestinal conditions that would be aggravated by slowed gastric emptying, needs individual assessment. Kidney function, gallbladder health and any concurrent medicines all form part of the picture.

One practical note on contraception: for tirzepatide specifically, the MHRA advises that women taking oral contraceptives add a barrier method for the first four weeks of treatment and for four weeks after each dose increase. Absorption of the pill may be reduced. This is the kind of detail that a prescriber flags as a matter of course, it often gets missed when someone sources a medicine without clinical oversight.

Our prescribers at nume review each consultation personally. If there is any uncertainty about suitability, that is worked through before a prescription is issued, not after.

What are the side effects that actually affect most people day to day?

Nausea is the most frequently reported experience, particularly in the early weeks. Some people also notice loose stools, constipation, indigestion or burping. Headaches and fatigue appear in a smaller proportion. Injection-site redness or mild discomfort is possible but usually brief.

For the majority of people, these effects are front-loaded. They tend to peak in the first fortnight after starting or increasing a dose, then ease. Eating smaller, lower-fat meals and staying well hydrated helps most people get through the adjustment period. Persistent or severe symptoms are worth discussing with your prescriber; dose timing and food choices can make a real difference.

The NHS medicines page for tirzepatide lists the full reported frequency breakdown, it is worth reading alongside your Patient Information Leaflet. For a broader overview of what living with these medicines looks like in practice, the weight loss injection guide covers the day-to-day questions our prescribers hear regularly.

It is worth spending sixty seconds on one check before you start: confirm the pharmacy you are using appears on the GPhC register. Counterfeit pens (sold via social media and unverified websites) have caused genuine harm in the UK, and if you are wondering whether it is safe to take injections for weight loss from an unverified source, the answer is clearly no. Verified supply is not an optional extra; it is the foundation of safe treatment.

When should you seek urgent medical help?

Most side effects can wait for your next scheduled prescriber contact. A few cannot. In January 2026 the MHRA issued a formal Drug Safety Update highlighting acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. The symptom to recognise is severe, persistent abdominal pain that radiates through to the back, with or without vomiting. If that happens, stop the injection and seek emergency help immediately. Do not wait to see if it passes.

Other situations that need prompt medical attention: signs of a serious allergic reaction (swelling of the face, throat or tongue, difficulty breathing, rapid heartbeat), symptoms of severe dehydration from prolonged vomiting or diarrhoea, and (for anyone with diabetes) signs of hypoglycaemia if combined with certain other medications. Gallbladder symptoms, including sudden upper-right abdominal pain, also warrant same-day contact with a healthcare professional.

If you are unsure whether a symptom is urgent, the default is to act. Contact your prescriber, call 111, or go to A&E depending on severity. You can report any suspected side effect, including ones you are uncertain about, through the MHRA Yellow Card scheme, reports from patients contribute to the ongoing safety monitoring of these medicines.

Our FAQs cover many of the practical questions that come up during treatment, including whether the weight loss injection is safe for people with specific health histories, and a closer look at whether weight loss injections are safe across the range of options currently available. Patients also frequently ask whether the new weight loss injection is safe, which is answered in detail for those considering tirzepatide for the first time. And if you have a specific concern before or during a course, the contact page connects you with the aftercare team, seven days a week. Safety questions never have to wait until Monday.