The Worse Side Effects of Weight Loss Injections — and When They Actually Matter

You've just started your first pen. Here's what's probably happening

Picture this: it's day five of your first week, you've had a smaller appetite than usual, and last night's dinner didn't sit well. You're wondering whether this is normal or a sign something's wrong. The honest answer is that it's almost certainly normal, and almost certainly temporary.

The most frequently reported side effects of GLP-1 and dual GIP/GLP-1 medicines are gastrointestinal. Nausea tops the list, followed by loose stools or diarrhoea, constipation (sometimes these alternate), indigestion, burping and an uncomfortable bloated feeling after eating. Fatigue, mild headaches and dizziness also appear in the early weeks for some people. These happen because the medicines slow how quickly food leaves your stomach, which is exactly the mechanism that reduces hunger, but your gut takes a little time to adjust.

Injection-site reactions are worth mentioning too: a small red mark, mild itching or brief tenderness where the needle went in. Rotating between sites (abdomen, thigh, upper arm) keeps this to a minimum. If you want a fuller picture of what the side effects of weight loss injections actually look like in practice, including injection technique and site hygiene, that guide covers the basics alongside the wider clinical picture.

For the vast majority of people, this early-weeks discomfort settles. Eating smaller portions, avoiding rich or very fatty meals, staying well hydrated and eating slowly all help. The starting dose of tirzepatide is 2.5mg specifically because it gives your system time to settle before any therapeutic increase, the lower doses exist for tolerability, not hesitation.

Side effects that need a call to your prescriber or GP

Not everything should be waited out. Some symptoms are a signal to get advice the same day, not panic, but don't dismiss them either.

Persistent vomiting or diarrhoea lasting more than a day or two raises the risk of dehydration. That in turn can affect kidney function, and if you take ACE inhibitors, angiotensin-receptor blockers or diuretics for blood pressure, the combination deserves prompt attention. Contact your prescriber or GP if you can't keep fluids down for more than 24 hours.

Gallbladder problems are another known association. Rapid weight loss of any kind can increase the risk of gallstones, and GLP-1 medicines slow gallbladder emptying. Symptoms to watch for: pain in the upper right abdomen, nausea alongside it, pain that moves to your shoulder. These warrant a call rather than a wait-and-see.

Mood changes (feeling unusually low, anxious or, in rare cases, having thoughts of self-harm) should be taken seriously and reported to a clinician straightaway. The evidence here is still being studied, but the precautionary guidance is clear: tell someone.

Allergic reactions, though uncommon, can include swelling around the face or throat, rash or breathing difficulty. That's a 999 situation. And if you're using tirzepatide and are also taking an oral contraceptive pill, be aware that absorption of the pill can be reduced in the first four weeks of treatment and for four weeks after each dose increase, our guide to how these medicines affect the body covers that in more detail.

The symptom that needs emergency attention: pancreatitis

There's a misconception that stomach discomfort on these medicines is always just the standard nausea, and it will pass. Usually, it will. But one pattern of abdominal pain is categorically different: severe, persistent pain in the upper abdomen that radiates through to the back, often accompanied by vomiting and a sense that the pain doesn't ease when you change position.

That description fits acute pancreatitis, inflammation of the pancreas. It's uncommon, but it can be serious. In January 2026 the MHRA issued a Drug Safety Update specifically for GLP-1 receptor agonist medicines, naming acute pancreatitis as a known but infrequent risk that the regulator wanted patients and clinicians to take seriously. You can find that communication via the MHRA Drug Safety Updates page.

If you develop that kind of pain (severe, persistent, radiating to the back) stop the injection and go to A&E or call 999. Do not wait to see if it settles. Pancreatitis needs imaging and clinical assessment; it isn't something to manage at home while you finish the week.

If you've experienced any unexpected symptom you think might be linked to your medicine, the right thing to do is report it through the Yellow Card scheme. It takes five minutes and contributes to the ongoing monitoring of medicines in the UK. Our prescribers actively encourage patients to use it.

Side effects that are often overstated, and why clinical support changes the picture

People sometimes come to a consultation having read horror stories online and expecting weeks of misery. The clinical trial data tells a more measured story. Symptoms are common, particularly in the first month; serious adverse events are rare; and most people who persist past the adjustment phase report significantly reduced appetite with a side-effect burden that's become manageable.

That said, the picture is genuinely individual. Your other medications, your medical history, how quickly you titrate and how you eat all influence what you experience, and reading up on the full range of side effects of weight loss injections before you begin can help you distinguish what's expected from what needs attention. A common question at this stage is whether weight loss injections speed up metabolism, and understanding the mechanism helps explain why the side-effect profile varies so much between people. That's why these medicines are prescription-only with ongoing clinical review, not something to start, adjust or stop based on what a forum says.

At nume, a clinician reviews your answers before any treatment is issued, and aftercare is available seven days a week, not just at sign-up. If something feels wrong mid-treatment, our prescribers are there. A free consultation is the place to raise concerns about specific side effects before you start, not after. Our clinical team discusses suitability and the likely side-effect profile as a standard part of that conversation, because knowing what to expect, and what to watch for, makes a material difference to how well people manage.

If you're still weighing up what these medicines involve more broadly, the weight loss injections overview is a good starting point, and the FAQs cover many of the questions our prescribers hear most often.