Side effects, plainly explained.

The common side effects

Mounjaro (tirzepatide), Wegovy and the Wegovy Pill (semaglutide) slow the emptying of the stomach and act on appetite. Most side effects come from the gut, and they are most noticeable in the first weeks and for a few days after each dose increase. For most people they ease as the body adjusts.

• Nausea, and sometimes vomiting
• Diarrhoea or constipation
• Indigestion, heartburn, burping and bloating
• Stomach pain or discomfort
• Tiredness, headache or dizziness, often linked to eating and drinking less
• Injection-site redness or itching (injections only)
• Hair thinning, which has been reported and is often linked to fast weight loss

Making them easier

• Eat smaller meals and stop when you feel full. Overeating is the most common trigger for nausea.
• Go easy on fatty, fried and very sugary food, especially on injection day and the day after.
• Drink water steadily through the day. Vomiting or diarrhoea can dehydrate you quickly.
• For constipation, add fibre gradually and keep drinking fluids.
• Don't step up a dose until the side effects of the current one have settled. Your prescriber can hold you on a dose for longer.
• Tell us if a side effect is stopping you eating or drinking normally. That is a reason to review the dose, not to push on.

Warning signs that need help now

Stop the medicine and get urgent medical help (call 111, or 999 in an emergency) if you have:

• Severe stomach pain that doesn't go away, sometimes spreading to your back, with or without vomiting. This can be pancreatitis.
• Pain in the upper right of your tummy, a high temperature, or yellowing of the skin or eyes. These can be gallbladder problems, which are more likely during fast weight loss.
• Signs of dehydration after vomiting or diarrhoea: very dark urine, passing little urine, dizziness or confusion. Severe dehydration can affect the kidneys.
• Signs of a serious allergic reaction: swelling of the face, lips or throat, difficulty breathing, or a widespread rash.
• Low blood sugar (shakiness, sweating, confusion) if you also take insulin or a sulfonylurea such as gliclazide.

Pregnancy, breastfeeding and contraception

GLP-1 medicines must not be used during pregnancy, while breastfeeding, or while trying to get pregnant. If you could become pregnant, use effective contraception throughout treatment.

Mounjaro can make the contraceptive pill less effective in people who are overweight. If you take an oral contraceptive, the MHRA advises also using a non-oral method, such as condoms, for four weeks after starting Mounjaro and for four weeks after each dose increase, or switching to a non-oral method.

If you are planning a pregnancy, stop Mounjaro at least one month before trying to conceive, and Wegovy or the Wegovy Pill at least two months before. If you become pregnant while taking any of them, stop and contact your prescriber.

Alcohol

There is no specific interaction between these medicines and alcohol, but alcohol can make nausea worse, adds calories, and raises the risk of low blood sugar if you also take diabetes medicines. Many people find they want less of it. If you drink, keep within the UK guidance of no more than 14 units a week, spread over several days.

Other medicines and surgery

• Tell the prescriber about every medicine you take. Because these medicines slow stomach emptying, they can change how some tablets are absorbed.
• If you take insulin or a sulfonylurea, your diabetes medicines may need adjusting to avoid low blood sugar.
• Before any operation or procedure under general anaesthetic or deep sedation, tell the team you take a GLP-1 medicine. The MHRA has warned that food can stay in the stomach despite fasting, which raises the risk of breathing it in.

Orlistat and Alli

Orlistat works differently: it stops about a quarter of the fat you eat being absorbed. Its side effects follow from that, and they are worse after fatty meals.

• Oily spotting, wind with an oily discharge, urgent or more frequent bowel movements, and oily or fatty stools
• Keep fat to about a third of the calories in each meal to reduce these effects
• Take a multivitamin at bedtime, at least two hours after orlistat, because it can reduce the absorption of vitamins A, D, E and K
• Severe diarrhoea can stop the contraceptive pill working; use an extra method if this happens
• Tell the prescriber if you take levothyroxine, warfarin, ciclosporin or medicines for epilepsy

Reporting a side effect

You can tell us about any side effect, and we can help you report it. You can also report it yourself to the MHRA through the Yellow Card scheme. Reports help the regulator spot problems with medicines.

For anything on this page that applies to you, the patient information leaflet in your pack is the authoritative source. Our treatment pages explain each medicine in more detail.