Weight Loss Injections — Are They Safe? What the NHS Evidence Actually Shows

What side effects do weight loss injections actually cause?

The most common complaints are gastrointestinal: nausea, vomiting, diarrhoea, constipation, indigestion and burping. These tend to be most noticeable in the days after starting treatment or stepping up to a higher dose, and for many people they settle within one to two weeks as the body adjusts. Fatigue, headache and dizziness also appear in trial data, as do mild reactions at the injection site itself.

The NHS medicines page for tirzepatide lists these effects clearly and is worth bookmarking, it is updated as new information emerges. The semaglutide equivalent covers Wegovy's profile in the same format. Reading it takes under a minute and gives you a solid baseline before your consultation.

Neither medicine is without risk, and the licensed doses are titrated slowly for exactly this reason: starting low gives the body time to adapt and keeps side effects manageable for most people. Tolerability varies, some patients sail through; others find the first month genuinely uncomfortable. Honest prescribing involves discussing both possibilities upfront, which is what our clinical team does as standard.

When do weight loss injections become genuinely dangerous?

Most side effects are unpleasant rather than dangerous. A smaller number of signals require prompt action. In January 2026 the MHRA published a Drug Safety Update on GLP-1 medicines, highlighting acute pancreatitis: it is infrequent, but it can be serious. The warning sign is severe, persistent abdominal pain that spreads toward the back, with or without vomiting, if that happens, stop the injection and seek urgent medical help rather than waiting to see whether it passes.

Other situations that need prompt attention include gallbladder symptoms (right-side upper abdominal pain, fever, jaundice), signs of significant dehydration following severe vomiting or diarrhoea, any allergic reaction (swelling, breathing difficulty, rapid heartbeat), and changes in mood or thoughts of self-harm. People with a history of medullary thyroid carcinoma or MEN2 syndrome should not take these medicines at all, these are absolute contraindications picked up during the prescribing assessment.

If you experience anything that concerns you between doses, the right first call is to the prescriber or pharmacist who issued the prescription, not a search engine. Our aftercare team is available seven days a week for exactly this reason. You can also report suspected side effects directly through the MHRA Yellow Card scheme, patient reports genuinely inform post-market safety monitoring.

What does the NHS say about who these injections are safe for?

The NHS does not recommend weight loss injections for everyone who wants to lose weight. Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition such as high blood pressure, high cholesterol or type 2 diabetes. Lower thresholds apply for some ethnic backgrounds under UK guidance. They are not licensed for under-18s, and they are not recommended during pregnancy, while breastfeeding, or for anyone actively trying to conceive.

NHS England's guidance on weight management injections also highlights specific precautions around contraception: women taking oral contraceptive pills who start tirzepatide should use an additional non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because gut-slowing effects can reduce pill absorption. Transdermal HRT may be worth discussing with your GP for the same reason.

Surgery is another consideration that is easy to overlook: anyone scheduled for a procedure should tell their anaesthetist and surgical team that they are using a GLP-1 medicine, as gastric emptying changes affect anaesthetic safety. These are the kinds of detail that a prescriber reviews with you, not things that should fall through the cracks of an unsupervised online order. If you are weighing up whether weight loss injections are available through the NHS, including how NHS access works alongside private routes, that page covers eligibility and the referral process in more depth. For a broader picture of how weight loss injections work alongside these precautions, that page covers the mechanism and monitoring in more depth.

Is buying a weight loss injection online safe, and how do you check?

It depends entirely on where you buy it. The MHRA has seized roughly 20 million doses of illegally traded weight loss medicines worth around £45 million since enforcement intensified in 2025, including the first UK site manufacturing counterfeit tirzepatide pens, raided in Northampton in October 2025. Fake pens have been found to contain insulin rather than the labelled medicine, an error that is life-threatening in someone without diabetes.

The practical check takes seconds: go to the GPhC pharmacy register and search the pharmacy's name or registration number. Legitimate UK online pharmacies are listed there. If a seller is not on the register, do not order from them, regardless of how professional their website looks. Red flags include prices far below the current market range, offers of these medicines without a clinical assessment, and discount codes, for a prescription medicine, that combination is a signal, not a bargain.

nume operates from a GPhC-registered pharmacy (registration number 9012878, verifiable on the register). Every prescription is reviewed by a GPhC-registered Independent Prescriber (a real clinician, not automated software) before anything is dispensed. Patients often ask whether weight loss injections are considered safe by the NHS, and that page sets out what the NHS evidence review concludes, including which patient groups the guidance is most relevant to. There is also a dedicated page on whether the weight loss injection is safe according to NHS guidance, which looks at the question from the perspective of individual medicines rather than the category as a whole. If you want to understand more about NHS access routes versus private options, that page walks through both routes and their respective criteria. For a fuller look at weight loss treatment options more broadly, including how these medicines sit alongside other approaches, that overview covers the landscape.