What are the side effects of weight loss injections?

1. The first days: your digestive system is the first to notice

Weight loss injections work partly by slowing how quickly your stomach empties food into the small intestine. That mechanism is behind most early-stage side effects. Nausea is the most frequently reported, and it tends to peak in the 24–48 hours after each injection before fading. Vomiting, loose stools or constipation can follow the same pattern. Burping and indigestion are also common, less dramatic but persistent enough to catch people off guard.

For the injectable GLP-1 medicines licensed in the UK, the starting dose is deliberately low: tirzepatide begins at 2.5 mg and semaglutide at 0.25 mg, not because those doses drive significant weight loss, but because they give your system a gentler introduction. Most people find GI effects are worst in those opening weeks, then ease off considerably. Eating smaller portions and avoiding very fatty or rich food in the hours around your injection day helps many people.

Fatigue and headache appear for some patients in the early phase too, often tied to reduced calorie intake rather than the medicine directly. Dizziness and injection-site reactions (a small patch of redness or tenderness where you injected) round out the typical early picture. The NHS tirzepatide medicines page lists these in full and is a reliable reference to keep to hand.

2. As your dose increases: what changes and what stays the same

Most people tolerate the starter dose reasonably well and then notice side effects flaring again each time the prescriber increases the dose. This is expected. The GI pattern from week one tends to repeat, briefly, at each step, nausea for a few days, then settling. Slow, supervised titration exists precisely to manage this; your prescriber monitors how you're tolerating each level before advising a change.

Beyond the digestive effects, some people notice hair thinning during active weight loss. This is typically telogen effluvium (a temporary shedding response to rapid calorie deficit) rather than a drug-specific toxicity. Adequate protein intake matters here, and it's worth raising with your prescriber if it's concerning you. Fatigue can also return at a higher dose, again linked to appetite suppression and overall intake more than the medicine itself.

Injection technique becomes important for comfort over time. Rotating sites (abdomen, thigh, upper arm) prevents local skin changes. Keeping your pen in the fridge door (2–8 °C, per the manufacturer's storage guidance) and taking it out 30 minutes before injecting reduces stinging at the site. A broader look at side effects across different weight loss injections covers what varies between tirzepatide and semaglutide in more detail.

3. Rare but serious: the side effects that need urgent attention

Most people on GLP-1 weight loss injections tolerate them well long term. But some adverse effects are serious and require prompt action. Know these before you start.

Pancreatitis. In January 2026, the MHRA issued a Drug Safety Update confirming acute pancreatitis as a known (though infrequent) risk with GLP-1 medicines. The warning sign is severe, persistent stomach pain that may spread through to your back, sometimes with vomiting. Stop taking the injection and seek emergency care. Do not wait to see if it passes.

Gallbladder problems. Rapid weight loss of any kind can increase the risk of gallstones. Symptoms include sharp pain in the upper right abdomen, particularly after eating, and possibly fever or jaundice. These need medical evaluation promptly.

Severe allergic reactions. Swelling of the face or throat, difficulty breathing or a widespread rash after an injection are signs of an allergic reaction. Call 999.

Mental health changes. Some people report low mood or, in rare cases, thoughts of self-harm. If you or someone close to you notices this, contact your prescriber or GP straight away. Information on more serious reported effects is available if you want to read further before your consultation.

You can report any suspected side effect of a weight loss injection (however minor) using the MHRA's Yellow Card scheme. Reports from patients help regulators track patterns that emerge in real-world use.

4. When to get medical help: a clear checklist

This is the part to screenshot or share with someone who'll be around when you're injecting. Get help (from a GP, pharmacist, 111, or 999 depending on urgency) if you experience:

• Severe or persistent stomach pain, especially pain radiating to the back
• Signs of dehydration after vomiting or diarrhoea (dark urine, dizziness on standing, rapid heartbeat)
• Upper-right abdominal pain with fever
• Swelling around the face, lips or throat
• A racing or pounding heartbeat that comes on suddenly
• Sudden changes in vision
• Low mood, thoughts of self-harm or any concerning mental health change

For questions that are urgent but not emergencies, NHS 111 is the right first call. For anything that can wait, your prescriber is the right person, every repeat prescription at our treatment consultation is reviewed by a GPhC-registered Independent Prescriber who can advise on side effects as they change over time. Our support team is also available seven days a week for aftercare questions between prescribing reviews.

The effects of weight loss injections on the body are real, and being prepared makes a practical difference. A concise summary of whether side effects are worth the tradeoff (and what most patients actually experience long term) is worth reading alongside this page, and if you are based locally you may also find our guide to weight loss injections in Essex useful for understanding how to access supervised treatment close to home.