Weight-Loss Injection Side Effects: What's Common, What's Serious, and When to Get Help

Sep 30

4 min read

Updated: Oct 10

A calm, photo-realistic glass of water and a blank notebook with a pencil on a wooden bedside table, representing tracking symptoms and staying hydrated

Weight-loss injections are now used by hundreds of thousands of people in the UK. Most people who take them do well. But like every medicine, they can cause side effects. Some are common and usually mild. A few are rare but serious.

Knowing the difference helps you stay safe. Let's go through it step by step.

Why do these medicines cause side effects?

These medicines copy a hormone called GLP-1 that your gut makes after you eat. (Some newer ones also copy a second gut hormone called GIP.) These hormones tell your brain you are full. They also slow down how fast food leaves your stomach.

That slower stomach is part of why you feel full for longer. But it is also why many side effects happen in the tummy and gut.

The common side effects

The UK medicines regulator, the MHRA, collects reports of side effects through its Yellow Card scheme. Between 2020 and 2025, around half of all the side effects reported for these medicines were gut-related.

The most common ones are:

Feeling sick (nausea) and sometimes being sick
Diarrhoea or constipation
Bloating, wind and burping
Heartburn (a burning feeling in your chest after eating)
Headaches, tiredness and dizziness, often linked to not eating or drinking enough

These side effects are most likely when you first start, or just after your dose goes up. For many people they ease off after a few days or weeks as the body gets used to the medicine. That is why doses are usually increased slowly, one step at a time.

Interestingly, a 2026 study in the journal Nature found that some people's genes may make them more likely to feel sick on these medicines. So if you get nausea and a friend doesn't, it is not your fault.

Simple ways to cope

NHS obesity teams, such as the one at Cambridge University Hospitals, share practical tips like these:

Eat smaller meals, slowly. Stop when you feel full, even if there is food left.
Choose bland, low-fat foods such as toast, crackers or soup if you feel sick.
Don't lie down straight after eating.
Drink plenty of fluids. These medicines can make you feel less thirsty as well as less hungry. The Cambridge team suggests about two to three litres of water or sugar-free drinks a day.
Keep things moving. Fibre, fluids and a daily walk help with constipation.
Go easy on fizzy drinks, spicy food, fatty food and caffeine if you have wind or heartburn.

If side effects don't settle, or they stop you eating and drinking properly, speak to the clinician who prescribed your medicine. They may keep you on your current dose for longer before increasing it.

One extra thing to know: the pill

These medicines can stop some tablets being absorbed as well as usual, especially during the first weeks and after each dose increase. For tirzepatide, NHS advice is to also use a barrier method, like condoms, for four weeks after starting and after each dose increase if you rely on the contraceptive pill. Ask your pharmacist or prescriber what applies to you.

The rare but serious side effect: pancreatitis

The pancreas is an organ behind your stomach that helps you digest food. Pancreatitis means it has become swollen and inflamed. It is rare, but it can be very serious.

In January 2026, the MHRA strengthened its warnings about this. It said that "for the vast majority of patients" these medicines are safe and effective. But it also shared that it had received more than 1,000 Yellow Card reports of pancreatitis linked to this group of medicines, including a small number of deaths.

The MHRA is also running a project called the Yellow Card Biobank. It is looking at whether some people's genes make them more likely to get pancreatitis, so doctors can spot who might be at higher risk in future.

When to get help urgently

Stop and get medical help straight away if you have:

Severe tummy pain that won't go away, especially if it spreads to your back, with or without being sick. This can be a sign of pancreatitis. Contact your clinician urgently, or call NHS 111.
Signs of a severe allergic reaction, such as swelling of your face, lips, tongue or throat, or trouble breathing. Call 999.
Signs of dehydration, such as being very sick or having diarrhoea for a long time, feeling faint, or passing very little urine.

If you see a doctor, A&E or NHS 111, tell them you are taking a weight-loss injection, even if you get it privately. Private prescriptions don't always appear on your NHS GP record, and the MHRA has specifically asked clinicians to ask about them.

Report side effects with a Yellow Card

Anyone can report a suspected side effect to the MHRA through the Yellow Card scheme, online or through the Yellow Card app. You don't need to be sure the medicine caused it. Every report helps the MHRA spot safety problems sooner.

The bottom line

Side effects from weight-loss injections are common, but most are mild and short-lived, and there is a lot you can do to manage them. A few are rare but serious, so it is worth knowing the warning signs.

These are prescription-only medicines. They should only be started after a proper assessment by a qualified clinician, who can check they are right for you and support you through any side effects along the way.