GLP-1 side effects: from the very common to the very rare

Between 2007 and October 2025, the UK medicines regulator received 1,296 Yellow Card reports of pancreatitis in people taking a GLP-1 medicine. Nineteen were fatal. The MHRA published those figures in January 2026, when it strengthened the warnings on every product in the class, and it set them against a second number: roughly 25.4 million packs dispensed over five years. Its message came in two parts: pancreatitis is a recognised reaction that can, in rare cases, be severe, and its overall frequency remains uncommon. Most GLP-1 side effects are far more ordinary, and far more frequent.

This guide starts from the official texts. We reread the Summary of Product Characteristics and the patient leaflets for Wegovy (semaglutide), Mounjaro (tirzepatide) and Ozempic (semaglutide) as published in the UK, then the two large obesity trials and the MHRA's safety communications. It gives the overview for the class; Wegovy and Mounjaro each have an article of their own.

Why the stomach speaks first

The most common effects are digestive, and that follows from how the drugs work. These medicines slow the emptying of the stomach and act on the brain's appetite centres: the meal stays put for longer, hunger recedes. The Wegovy product information actually includes "delayed gastric emptying" in its own list of side effects.

Nausea, diarrhoea, vomiting, constipation and abdominal pain make up the usual picture. The three product summaries agree on the timing: they occur mainly while the dose is being stepped up, then ease. The Wegovy summary adds that most episodes are mild to moderate and short-lived.

One consequence is worth knowing. Vomiting and diarrhoea cost the body water, and the leaflets ask patients to drink plenty to avoid dehydration, with particular care for anyone whose kidneys are fragile.

GLP-1 side effects ranked by frequency

Medicine leaflets in the UK all use the same scale. "Very common" means an effect seen in more than 1 person in 10; "common", up to 1 in 10; "uncommon", up to 1 in 100; "rare", up to 1 in 1,000; "very rare", up to 1 in 10,000. When the data do not allow an estimate, the leaflet says "not known".

Here is how the three medicines classify the effects people ask about most.

This table does not rank the medicines against each other. The frequencies come from different trials in different people: adults with type 2 diabetes for Ozempic, adults living with obesity or overweight for Wegovy, both for Mounjaro. The same molecule, semaglutide, does not even get the same classification in its two sets of product information.

What the two big trials show

In the trials, digestive problems affect a large share of participants, but few of them stop treatment because of it.

STEP 1, published in the New England Journal of Medicine in 2021, followed 1,961 adults without diabetes for 68 weeks. Its authors describe nausea and diarrhoea as the most frequent events on semaglutide, usually passing. Stopping treatment for digestive reasons concerned 4.5% of treated participants, against 0.8% on placebo. The detail, effect by effect, is in our article on Wegovy side effects.

SURMOUNT-1, which appeared in the same journal in 2022, enrolled 2,539 adults on tirzepatide or placebo for 72 weeks. Same finding: mostly digestive effects, mild to moderate for the most part, concentrated in the dose-escalation period. Depending on the group, 4.3 to 7.1% of treated participants stopped because of an adverse event, against 2.6% on placebo. We come back to it in the article on Mounjaro side effects.

A trial describes a selected population, closely monitored. It is good at showing what is common and poor at showing what is rare. That is the job of safety monitoring, which carries on after a medicine reaches the market. In the UK it runs through the Yellow Card scheme.

What owes as much to the weight loss as to the drug

Some of the listed effects go with rapid weight loss, whatever its cause. Gallstones are the best-documented example: the Mounjaro summary notes that in its weight management studies, acute gallbladder events were positively correlated with the amount of weight lost.

Hair loss follows the same logic. In the Wegovy trials it was reported by 2.5% of treated participants against 1.0% on placebo, more often in those who had lost at least 20% of their weight. Most recovered while staying on treatment. The hollowed look some people notice in the mirror belongs here too, though no leaflet lists it; we look at the evidence in our article on Ozempic face.

The heart, lastly, beats a little faster. The Wegovy documents report a mean rise of 3 beats per minute in the trials, and the product information lists "increased heart rate" among the uncommon effects.

Rare, but worth knowing

Four serious effects recur in the product information. Their rarity is no reason not to know how to recognise them.

Acute pancreatitis, an inflammation of the pancreas, is classed as "uncommon" for all three medicines. In the Wegovy trials it was confirmed in 0.2% of treated participants and fewer than 0.1% of those on placebo. The MHRA's January 2026 update added that, among the reports it has received, some cases were necrotising or fatal, which is why the wording was strengthened across the class.

Severe allergic reactions, such as anaphylaxis, are classed as "rare".

The optic nerve condition known as NAION is the most recent addition. In June 2025 the European Medicines Agency's safety committee concluded that it is a "very rare" side effect of semaglutide, and the UK product information for Wegovy and Ozempic now lists it. According to the studies reviewed, the risk appears to be roughly doubled in adults with type 2 diabetes who take the drug, which works out at about one extra case per 10,000 people treated for a year. The Mounjaro product information does not mention it.

Bowel obstruction, finally, appears in both sets of semaglutide product information with a frequency "not known", on the basis of reports received after marketing.

Two situations concern people with diabetes. Combining these medicines with a sulfonylurea or insulin raises the risk of hypos, and it is for the doctor to adjust those treatments. Complications of existing diabetic retinopathy are classed as "common" in the semaglutide product information.

One last point applies to everyone: before a general anaesthetic or deep sedation, tell the team you are on a GLP-1 medicine, because a stomach that empties slowly may still contain food. The MHRA gives the same advice ahead of any surgery.

The symptoms that mean getting help now

The leaflets are specific about the symptoms that should not be managed alone. Here they are, with the action each one calls for.

In an emergency, call 999 or go to A&E. For everything else, such as vomiting that will not stop, being unable to keep fluids down or feeling faint on standing, the right move is to ring NHS 111, your GP or your prescriber, or to speak to a pharmacist, without waiting for the next review. If you buy your treatment privately, tell NHS staff: a private prescription may not show on your GP record.

What the MHRA is watching

The regulator has issued a steady run of guidance since the weight management licences arrived.

In June 2025 it turned to contraception and pregnancy. GLP-1 medicines should not be used during pregnancy, while trying to conceive or while breastfeeding, and effective contraception is advised throughout treatment. Mounjaro has a specific warning: it may make the contraceptive pill less effective in women who are overweight, so a barrier method is advised for four weeks after starting and after each dose increase. Depending on the medicine, treatment should stop one to two months before a planned pregnancy.

In the same guidance the MHRA warned against buying these medicines from unregulated sellers such as beauty salons or via social media. They are prescription-only and should come from a registered pharmacy after a consultation.

In January 2026 came the pancreatitis update described above. Alongside it, the Yellow Card Biobank, run with Genomics England, is recruiting people who were admitted to hospital with pancreatitis while on a GLP-1 medicine, to find out whether genes influence the risk.

Other regulators add to the picture. In France, a national safety review published in January 2026 flagged a possible signal of nutritional deficiencies linked to rapid weight loss, including iron deficiency and loss of muscle; the agency describes it as a signal to investigate, not an established risk. As for suicidal thoughts, the European Medicines Agency's safety committee concluded in April 2024 that the available data did not establish a causal link with these medicines. Monitoring continues.

On the NHS, access is being phased in. NICE recommends semaglutide and tirzepatide for weight management alongside a reduced-calorie diet and increased physical activity, and GPs in England have been able to prescribe tirzepatide to priority groups since 23 June 2025.

At the table, a few simple rules

No food removes a side effect, and the leaflets give only one instruction: drink enough. The rest is the common sense that clinicians share. Smaller meals sit better in a slowed stomach. Eating slowly and stopping as soon as hunger fades avoids the over-full feeling. Very fatty or very large meals are often the worst tolerated on difficult days, and alcohol rarely helps, as we explain in Mounjaro and alcohol.

These are general pointers, not a protocol. If symptoms get in the way of eating for several days running, that is a question for your prescriber or pharmacist. Our guide to the GLP-1 diet goes further on what to put on the plate.

There is one thing the leaflets actively encourage: reporting. Any suspected side effect, including one not listed, can be reported by the patient directly through the MHRA's Yellow Card scheme, online or in the app. Recent additions to the product information grew out of exactly those reports.

Frequently asked questions

What are the most common GLP-1 side effects?

Digestive ones. Nausea and diarrhoea are classed as "very common" for Wegovy, Mounjaro and Ozempic, meaning they affect more than 1 person in 10. Vomiting, constipation and abdominal pain follow, with a classification that varies by medicine and by condition treated.

Do GLP-1 side effects go away?

For most people, yes. The product information states that digestive effects appear mainly while the dose is being increased and ease afterwards. Constipation tends to last longer than nausea.

Are GLP-1 medicines bad for the pancreas?

Acute pancreatitis is a recognised side effect, classed as "uncommon", so up to 1 person in 100. Severe, persistent stomach pain that may spread to the back needs urgent medical attention, and treatment should not be restarted if pancreatitis is confirmed.

Can GLP-1 medicines damage your eyesight?

An optic nerve condition, NAION, was added in 2025 to the product information for semaglutide medicines as a "very rare" effect. Any sudden loss of vision means contacting a doctor immediately. People with diabetic retinopathy are monitored separately.

Wegovy, Mounjaro or Ozempic: which has the fewest side effects?

The product information cannot answer that. The frequencies come from separate trials, with different doses and populations, and cannot be compared line by line. Choosing and adjusting a treatment is for the prescriber.

Do weight loss injections cause suicidal thoughts?

The European Medicines Agency's safety committee examined the question and concluded in April 2024 that the data did not establish a causal link. The subject remains under watch. Any unusual change in mood is worth raising with your doctor.

Can you take a GLP-1 medicine while pregnant?

No. The MHRA advises against use during pregnancy, while trying to conceive and while breastfeeding. Plan a pregnancy with your prescriber, and if you find you are pregnant during treatment, contact them without delay.

This article is for information only and is no substitute for advice from a healthcare professional. Never change your treatment without talking to your doctor.