Common side effects of weight loss injections: what the evidence says

What clinical trials recorded as the most frequent reactions

In the SURMOUNT-1 trial, which followed over 2,500 adults taking tirzepatide across 72 weeks, gastrointestinal events were the dominant category. Nausea was reported by a significant proportion of participants, particularly during the early titration period. Vomiting, diarrhoea, constipation, indigestion and burping also appeared frequently. The pattern for semaglutide is similar: the STEP 1 trial, published in the New England Journal of Medicine, found GI events were the most common reason participants reduced or paused their dose.

Beyond the gut, fatigue, headache, dizziness and injection-site reactions (redness, bruising or mild discomfort at the spot where the pen was used) are reported with some regularity. None of these are reasons to panic; all are listed in the approved Patient Information Leaflets and are expected parts of how these medicines work. Slowing gastric emptying, the mechanism behind much of the appetite reduction, is also the reason the gut reacts.

Timing matters here. Most people find the side effects are concentrated in the first one to two weeks after starting a new dose level, then fade. The licensed titration schedule (moving through dose steps gradually, with at least four weeks at each level) exists largely to give the body time to adapt. A prescriber who changes your dose more quickly than that is cutting against the grain of the safety data.

Reactions that need medical attention without delay

Most side effects from weight loss injections are an inconvenience rather than a danger, though a smaller set are not, and knowing the difference is the practical takeaway from this page.

In January 2026 the MHRA issued a formal Drug Safety Update on GLP-1 receptor agonist medicines, highlighting acute pancreatitis as an infrequent but potentially serious risk. The warning sign is severe, persistent abdominal pain that radiates toward the back, sometimes with vomiting. If that description fits what you are experiencing, stop the injection and contact your GP or NHS 111, or go to A&E if the pain is severe. Do not wait to see if it passes.

Gallbladder problems, including gallstones, have also been reported more frequently in people using these medicines than in the general population. Sudden pain in the upper right abdomen, especially after eating, is worth reporting to a doctor promptly. Signs of a serious allergic reaction (significant facial swelling, difficulty breathing, a widespread rash) require emergency care immediately.

Severe or prolonged vomiting and diarrhoea can lead to dehydration quickly, which in turn can affect kidney function. If you cannot keep fluids down for more than 24 hours, seek medical advice rather than waiting it out. Our prescribers are available through our aftercare team seven days a week if you have questions about what you are experiencing.

Injection-site reactions and practical ways to reduce common effects

Localised reactions at the injection site (redness, a small lump, mild itching) are common and almost always minor. Rotating between approved sites (abdomen, outer thigh, upper arm) on a consistent schedule keeps any one area from becoming irritated. Using a fresh needle each time and cleaning the skin beforehand are straightforward steps that reduce the risk; sterile alcohol swabs are designed for exactly this purpose.

For nausea specifically, eating smaller portions, avoiding very fatty or heavily spiced meals, and staying upright after eating all help. These are not medically complex adjustments, but they make a real difference for many people in the first few weeks. If nausea persists well beyond the initial adjustment period or is severe enough to affect daily life, that is a conversation to have with your prescriber rather than something to manage alone. The more serious reactions covered separately on this site include scenarios where continuing treatment needs clinical review.

One thing worth saying plainly: the side effects of weight loss injections vary considerably between individuals, so two people on the same dose can have entirely different experiences. What the trial data gives us is probabilities, not predictions. Your prescriber at nume considers your personal history (existing conditions, other medicines, previous responses) when assessing which treatment is appropriate and at what pace.

Reporting side effects and where to find further information

Every person taking a licensed medicine in the UK has the right to report suspected side effects directly to the MHRA. The Yellow Card scheme accepts reports from patients, not just clinicians, and the information genuinely shapes post-market safety monitoring. Tirzepatide and semaglutide both carry Black Triangle (▼) status, which means they are under additional surveillance, reporting unusual reactions is particularly valuable for these medicines.

For a full list of known side effects and contraindications, the authoritative source is always the Patient Information Leaflet included in the box with your medicine, or the relevant entry on the Electronic Medicines Compendium. No website, including this one, is a substitute for that document or for your prescriber's advice.

If you are considering starting treatment and want to understand how your individual circumstances affect the side-effect picture, the right step is a clinical consultation. Our prescribers talk through likely reactions, red flags and what to do if anything concerns you, as part of the free consultation before any prescription is issued. If you are weighing up your options, our guide to common weight loss injections sets out how the available treatments compare, and you can also read more about the Mojo weight loss injection if that is one you have come across in your research. If you have specific questions about the broader effects of these medicines, that page covers more ground.