Weight loss injections — how safe are they, really?

What side effects do weight loss injections actually cause?

The side-effect profile of both tirzepatide and semaglutide is well characterised. Across the major trials (SURMOUNT-1 for tirzepatide and STEP 1 for semaglutide) the picture is consistent: most people experience some gastrointestinal disruption, particularly in the early weeks. Nausea is the most common. Loose stools, constipation, indigestion, belching and reflux also appear regularly. Fatigue, headache and dizziness are reported less frequently.

These effects tend to peak when treatment begins or after each dose increase, then ease as the body adjusts. The slow titration schedule (starting at a low dose and stepping up gradually over weeks) exists largely because of this: it gives the digestive system time to adapt. Injection-site reactions (redness, mild swelling) are possible but rarely a reason to stop.

The NHS tirzepatide medicines page lists the full frequency breakdown. If a side effect is bothering you, the right move is to contact your prescribing team before adjusting anything. You can find a broader look at what these medicines do in the body at what weight loss injections do.

When should you seek urgent medical help?

Most side effects are unpleasant rather than dangerous. A few warning signs require prompt attention. Know them before you start, not after.

Severe stomach pain that radiates to the back (with or without vomiting) is the primary symptom of acute pancreatitis. This is uncommon, but the MHRA's January 2026 Drug Safety Update highlighted it explicitly for GLP-1 medicines: if that pain is persistent and severe, seek medical help the same day. Do not wait it out. Similarly, signs of significant dehydration from repeated vomiting or diarrhoea (dark urine, dizziness, very low urine output) warrant medical attention, particularly because dehydration can stress the kidneys.

Other symptoms to act on: signs of a serious allergic reaction (swelling of the face, lips or throat, difficulty breathing), gallbladder pain (sharp, sudden pain in the upper right abdomen), or any thoughts of self-harm or low mood that feels out of character. The MHRA's Yellow Card scheme lets patients report any suspected side effect directly, this is how post-market safety signals are tracked, and using it matters. You can also report concerns about suspect sellers through the same route. If you have questions about what to watch for, our support team is available seven days a week.

Are weight loss injections safe long-term?

This is the question that regulators, prescribers and patients are all watching closely. The licensed trials ran to 68 weeks (STEP 1, semaglutide) and 72 weeks (SURMOUNT-1, tirzepatide) (both concluded by the New England Journal of Medicine) and serious adverse events in those periods were low. Neither trial flagged a new or unexpected major safety concern at the end of the study period.

If you want a fuller picture before starting, our page on whether weight loss injections are safe covers the evidence in more detail, including what the trial data does and does not yet tell us. That is not a reason for alarm, many newer medicines carry this mark while more data accumulates. It is a reason to stay in contact with your prescriber, report anything unusual, and not treat these medicines as something to obtain and self-manage without clinical oversight.

People who should not use these medicines include those who are pregnant, breastfeeding, or trying to conceive; anyone under 18; and those with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome. Certain gastrointestinal conditions and a history of pancreatitis also require careful prescriber discussion. This list is not exhaustive, full contraindications are in each medicine's Patient Information Leaflet and on the eMC. The broader context of who qualifies and what the process looks like is covered on the weight loss injections overview page.

Does buying through a regulated pharmacy change the safety picture?

Yes, significantly. The MHRA has warned repeatedly about counterfeit weight loss pens sold without a prescription through social media and unverified online sellers. Fake pens have been found to contain the wrong substance entirely, in some cases, insulin. Regulatory enforcement in 2025 included raids and large seizures, including the discovery of an illicit UK manufacturing site in Northampton producing counterfeit tirzepatide pens. If you are researching your options locally, our weight loss injections service in Nottingham explains how we operate and what regulated supply looks like in practice, and our guide to whether the new weight loss injections are safe addresses the questions that come up most often around the newer medicines specifically. It is also worth reading our overview of what weight loss injections are if you are still building a picture of these treatments before committing to anything.

A GPhC-registered pharmacy dispenses only from the licensed UK supply chain. Every prescription at nume is reviewed by a GPhC-registered Independent Prescriber, not an automated system. Identity and current weight are verified before treatment is approved, and clinical review happens again before every repeat, meaning any side effects or concerns are assessed at the point of continuation, not ignored. You can confirm any online pharmacy's registration on the GPhC register. That check takes under a minute and is worth doing for any service you are considering. Our FAQs page covers common questions about the process if you want more detail before you start.