The real risks of weight loss injections, explained plainly

You've read the headlines — here's what the side effects actually look like day to day

Imagine picking up your first pen. The injection itself is usually fine, a short, fine needle into the abdomen or thigh. What catches most people off guard is the stomach. Nausea is the most frequently reported side effect of both tirzepatide (Mounjaro) and semaglutide (Wegovy), and it tends to peak in the days after a new or higher dose. Vomiting, loose stools, constipation, indigestion and burping are also common. Fatigue and headache turn up too, especially early on.

The good news is that these effects are usually temporary. Many people find they're most noticeable in the first one to two weeks at a new dose, then ease off. Eating smaller portions, avoiding very fatty or rich meals and staying well hydrated all help. For a detailed breakdown of what the clinical evidence shows, the NHS page on tirzepatide lists common, uncommon and rare effects with their approximate frequencies, which is worth bookmarking. More detail on the downsides of these medicines more broadly is covered on our guide to the cons of weight loss injections.

Injection-site reactions (redness, itching, a small lump) occur for some people. Rotating your injection site each week reduces the likelihood.

When weight loss injection risks become urgent: symptoms that need same-day medical attention

Most side effects from weight loss injections are uncomfortable rather than dangerous. A few are not.

Pancreatitis is a rare but serious risk. In January 2026 the MHRA issued a Drug Safety Update specifically highlighting acute pancreatitis as a known, infrequent but potentially serious effect of GLP-1 medicines. The symptom to know is severe stomach pain that doesn't ease up and may spread to your back, sometimes with vomiting. If that happens, stop your injection and go to A&E or call 999. Don't wait to see if it settles.

Other symptoms that warrant same-day contact with a doctor or 111: signs of a serious allergic reaction (swollen face, lips or throat, difficulty breathing, rapid heartbeat), symptoms of gallbladder trouble (sharp pain under the right ribs, fever, jaundice), or significant dehydration from persistent vomiting or diarrhoea, which can put strain on the kidneys. You can also read about the cancer risk questions that sometimes come up in relation to these medicines, which the evidence places in a specific and narrow context.

If you're unsure whether what you're experiencing needs urgent attention, NHS 111 is available online and by phone around the clock. Do not simply stop and restart the medicine without speaking to your prescriber first.

Who carries a higher risk, and what a prescriber checks before saying yes

Not everyone faces the same level of risk. A prescriber reviewing an application for Mounjaro or Wegovy will look carefully at your medical history before deciding whether treatment is appropriate. There are conditions where these medicines are not suitable: a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia type 2, certain inflammatory bowel conditions, and a history of pancreatitis all require careful clinical judgement.

Women who use the oral contraceptive pill and 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 the slowing of gastric emptying can reduce pill absorption. NHS guidance notes this applies specifically to tirzepatide; the equivalent evidence for semaglutide is less definitive. The NHS England page on weight management injections covers contraception and HRT considerations in full and is worth a read before you start.

If you're considering surgery, tell your anaesthetist and surgical team that you're on a GLP-1 medicine. Gastric emptying affects anaesthetic risk, and your team may ask you to pause treatment beforehand. That's a conversation to have with your prescriber well in advance, not on the morning of the procedure. For more on navigating the overall picture, our in-depth risk overview covers these factors in one place.

Reporting side effects and spotting a safe source

Both Mounjaro and Wegovy carry the Black Triangle (▼) designation, meaning the MHRA is actively gathering post-market safety data on them. If you notice a side effect you think might be linked to your injection, you can report it directly through the MHRA's Yellow Card scheme. Reports from patients, not just clinicians, are actively used to build the safety picture. It takes about two minutes.

A practical checking habit worth building: before every new pen, look at the liquid through the pen window. It should be clear and colourless. If it looks cloudy, discoloured or contains particles, don't use it and contact your pharmacy. This takes under a minute and is the simplest quality check you can do at home.

One thing the safety data consistently shows: the risks from fake or unverified pens are categorically worse than those from licensed medicines. The MHRA has warned publicly about counterfeit weight loss pens sold online, some of which were found to contain insulin, and our page on weight loss injection risks sets out what that means in practice. That's a reminder that where you get these medicines matters as much as which medicine you get. More on accessing treatment through a verified route is on our guide to getting weight loss injections safely.

If you'd like to understand the risks in the context of your own health, our prescribers discuss suitability and side effects as part of every consultation. Check your eligibility and a clinician will review your answers the same day, or visit our overview of weight loss injections to read more about how these treatments work before you do.