Side effects of weight loss injections: what the evidence says

What the trial data actually recorded for licensed weight-loss injections

The SURMOUNT-1 trial, published in the New England Journal of Medicine, followed 2,539 adults receiving tirzepatide over 72 weeks. Gastrointestinal events (nausea, diarrhoea, vomiting, constipation) were the most commonly reported adverse effects across all dose groups, and were the leading reason participants reduced or paused their dose. Crucially, most events clustered around the early weeks of treatment and after each upward titration step, rather than persisting throughout.

Semaglutide's phase-3 STEP programme told a similar story: the STEP 1 trial results, published in the NEJM, recorded nausea in roughly 44% of participants on 2.4mg, versus around 16% in the placebo group. Vomiting, diarrhoea and constipation followed the same gradient. Again, discontinuations due to GI events were a minority, and the slow titration schedule is specifically designed to reduce their severity. The NHS patient information page for tirzepatide lists the full side-effect profile in plain language and is a reliable starting point for anyone newly prescribed the medicine.

Other effects recorded in trials include fatigue, headache, dizziness and injection-site reactions (redness, bruising, itching at the pen site). Rotating between the abdomen, thigh and upper arm with each dose reduces local reactions; using a fresh needle for every injection matters for the same reason. None of these common effects signal that something has gone seriously wrong, but they are worth knowing about before you start, and if you are also considering nutritional supplements alongside treatment, it is useful to read about what side effects B12 injections for weight loss can bring so you can keep the two profiles distinct in your mind.

Side effects that do require prompt medical attention

The picture changes with a small set of rarer events. The MHRA's January 2026 Drug Safety Update singled out acute pancreatitis as a known risk with GLP-1 medicines: the regulator's guidance asks patients to seek urgent help for severe, persistent abdominal pain, particularly if it radiates to the back, with or without vomiting. This is not something to wait and see about.

Other situations that warrant same-day contact with a healthcare professional include: signs of dehydration (dizziness, very dark urine, no urination) caused by prolonged vomiting or diarrhoea; symptoms suggesting gallbladder trouble such as sharp pain in the upper right abdomen, fever or yellowing of the skin; and any signs of a serious allergic reaction, swelling of the face or throat, difficulty breathing, severe rash. People with a history of diabetic retinopathy who notice sudden vision changes should also seek prompt review, as rapid glucose fluctuation can occasionally affect the retina.

Low mood or thoughts of self-harm have been reported in post-market surveillance; while a causal link has not been established, it is worth mentioning at any point in treatment if your mood changes noticeably. The broader question of what side effects weight-loss injections carry covers this and other less-common reactions in more detail. If you are ever unsure whether a symptom is urgent, calling 111 is the right move.

Injection-site reactions and what reduces them

Because these medicines are given as weekly subcutaneous injections, the skin and tissue at the injection site matter more than people often expect. Injection-site reactions recorded in trials (redness, mild swelling, bruising, a brief stinging sensation) are generally minor and resolve without treatment. A few practical habits reduce them substantially.

Rotating the injection site systematically (never injecting into the same spot twice in a row), allowing the pen to reach room temperature before use, and cleaning the area beforehand with an alcohol swab all make a difference. Injecting into scar tissue or an area that is already bruised prolongs local irritation. The Patient Information Leaflet supplied with your pen covers the exact technique; the prescriber can also walk through it at any point, our prescribers discuss both technique and side-effect management as part of the consultation process.

Lipodystrophy (the thickening or hardening of subcutaneous tissue from repeated injections into the same spot) is well-documented with insulin therapy and less common with GLP-1 pens, but the rotation principle applies equally. The common conflation of these injections with insulin sometimes leads people to apply insulin injection advice directly; the pens and technique differ, so follow the guidance specific to your medicine.

How prescriber oversight and the consultation reduce risk

Side-effect risk is managed partly through the titration schedule (starting low, moving slowly) and partly through clinical review before and during treatment. At nume, a GPhC-registered prescriber reads every consultation on the same day it is submitted, a real clinician, not an automated screening tool. Before a dose increase, evidence of how the current dose has been tolerated is part of that review. That process exists specifically to catch problems early and adjust the plan.

The full side-effect guidance for weight-loss injections on our site goes into each category in more depth, and the FAQs page covers questions our prescribers hear regularly. If you are weighing up whether treatment is right for you, starting a free consultation is the way to have those questions answered against your own medical history. The prescribers who review at nume are also available through our aftercare service seven days a week for anything that comes up once treatment is underway, whether that's a question about a symptom that's appeared on a Saturday afternoon, or reassurance that a bout of nausea after the third pen is still within the expected pattern.

Suspected side effects (whether or not they turn out to be drug-related) can be reported via the MHRA's Yellow Card scheme. Reports from patients help regulators spot signals that trials cannot, and reporting is straightforward even without a formal diagnosis.