The real risks of weight loss injections — and when to seek help

You've started researching weight loss injections, and now you want the honest version

Most people arrive at this question after reading something alarming online — a headline about a hospital admission, a friend's bad week of nausea, a social media post about side effects nobody mentioned. That anxiety is understandable. These medicines are powerful, they affect your gut hormones, and they are not suitable for everyone. The good news is that the risk picture is genuinely well-characterised: tirzepatide (Mounjaro) and semaglutide (Wegovy) have been studied in large clinical programmes involving thousands of adults, and the data on side effects is detailed. Understanding what these injections actually are helps make sense of why the side effects take the shape they do, and a closer look at the three weight loss injections currently available can help you work out which one your prescriber is most likely to consider for you.

Both medicines work partly by slowing how quickly the stomach empties. That is why the side effect list is so heavily gastrointestinal. Nausea is the one people talk about most; it is also the one most likely to ease after the first week or two. Vomiting, diarrhoea, constipation, indigestion and burping are all on the list too. Headache, fatigue and dizziness feature less often. Injection-site reactions (mild redness or irritation) can happen with any subcutaneous injection. None of this is trivial when you are living through it, but for most people these effects are manageable and temporary, peaking around dose increases and then settling. The full risk profile for weight loss injections covers these patterns in more detail.

The side effects that need prompt medical attention

There is a meaningful difference between feeling queasy on day three and experiencing something that needs a doctor the same day. The MHRA's January 2026 Drug Safety Update on GLP-1 medicines flagged acute pancreatitis as an infrequent but serious concern, and the symptom to watch for is severe, persistent stomach pain that radiates towards the back, sometimes with vomiting. If that happens, stop eating, stop the injection and get medical help promptly. Do not wait to see if it passes on its own.

Gallbladder problems are another category worth knowing about: gallstones and gallbladder inflammation have been reported in clinical trials, and right-sided upper abdominal pain, especially after eating, warrants assessment. If you become severely dehydrated through persistent vomiting or diarrhoea, your kidneys can be affected, staying hydrated matters more on these medicines than people sometimes realise. Signs of an allergic reaction (swelling of the face or throat, difficulty breathing, rapid heartbeat) are rare but need emergency attention immediately. Some people also report low mood or, very rarely, thoughts of self-harm; these should be taken seriously and discussed with a clinician without delay. The section on serious risks and weight loss injections addresses some of these more extreme scenarios directly.

You can report any suspected side effect (including reactions you are not sure are related to the medicine) through the MHRA's Yellow Card scheme, which exists precisely to help regulators track signals from real-world use. Our prescribers will always ask you to report anything unexpected.

Who carries a higher risk, and what the prescriber checks for

Not everyone's starting point is the same. There are medical histories that put a person at higher risk from these medicines, and a thorough consultation is the mechanism for identifying them. A history of pancreatitis means the prescriber needs to weigh that carefully. Medullary thyroid carcinoma (or a family history of it) is a contraindication; so is Multiple Endocrine Neoplasia type 2. Severe gastrointestinal conditions such as gastroparesis need specific discussion. Pregnancy, breastfeeding and trying to conceive are all categories where these medicines are not recommended. They are not licensed for under-18s.

There is also the question of other medicines. Women taking the oral contraceptive pill should know that tirzepatide's effect on gastric emptying can reduce pill absorption, particularly during the first four weeks of starting treatment and for four weeks after each dose increase, adding a barrier method during those windows is the guidance from NHS England. If you take HRT, the same mechanism makes transdermal options (patches or gels) worth discussing with your doctor. Before any surgical procedure, your anaesthetist needs to know you are on one of these medicines. The weight loss injection overview covers eligibility in more depth, and our prescribers go through all of this systematically before treatment is ever approved. The NHS page on tirzepatide is a solid reference for the complete list of cautions, written for patients.

Making safety part of the routine, not an afterthought

The practical side of using these medicines safely is often underestimated. Storing the pen correctly is not optional, Mounjaro and Wegovy need to be kept refrigerated between 2°C and 8°C; many people keep theirs in the fridge door, which works well provided the temperature stays consistent. Room-temperature storage is only permitted for a limited window, and the exact timeframe is stated in the Patient Information Leaflet that comes with your pen, always check it rather than going by memory.

Rotating injection sites (abdomen, thigh, upper arm) reduces the chance of localised skin reactions. Starting at the lowest dose is not about being cautious for caution's sake; it gives your body time to adjust to the medicine before the dose increases, which is when side effects tend to peak again. Rushing that process creates more discomfort, not less. If you are unsure about anything during treatment (an unusual symptom, a missed pen, a planned holiday) the right place to start is a conversation with a clinician rather than a forum. At nume, aftercare is available seven days a week precisely because questions do not wait for office hours. Getting started with weight loss injections through a regulated pharmacy explains what the clinical pathway looks like from the first consultation onward. If you would like to talk through your specific situation and medical history, speak to our prescribers as part of a free consultation.