Mounjaro and Wegovy 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

How common each side effect is

The licensed product information for each medicine (the Summary of Product Characteristics, or SmPC) sorts side effects by how often they were seen in clinical trials. The table below follows those lists. The Wegovy Pill's list is the same as the Wegovy injection's, except that indigestion is very common and there are no injection-site reactions.

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.

How long side effects last

In the trials behind both medicines, nausea, vomiting and diarrhoea were most common while the dose was being increased, and became less common over time. Most were mild or moderate. They tend to come back for a few days after each step up, then settle again. Nausea is the most common: in the main Wegovy trials, 43.9% of people on the 2.4 mg dose had it at some point over 68 weeks, compared with 16.1% on placebo.

A minority of people stop because of them. In the main Wegovy weight-loss trials, about 4 in 100 people on the 2.4 mg dose stopped treatment because of stomach side effects. If side effects aren't settling, the usual answer is to stay on your current dose for longer, or go back down a step, rather than to stop altogether. That is a decision for your prescriber.

When side effects usually happen

Both injections start low and step up. Mounjaro starts at 2.5 mg, goes to 5 mg after four weeks, and then rises in 2.5 mg steps at least four weeks apart. Wegovy steps up every four weeks from 0.25 mg to 2.4 mg by week 17. The Wegovy Pill steps up at least a month apart, from 1.5 mg to 25 mg.

The SmPCs say nausea, vomiting and diarrhoea were seen mainly while the dose was going up, and for Wegovy, dizziness too. There is no fixed number of days a side effect lasts, but the pattern most people notice is below.

Tiredness and low energy

Feeling tired, weak or low on energy is a listed side effect of both medicines. On Wegovy and the Wegovy Pill it is very common, affecting more than 1 in 10 people. On Mounjaro it was common in the weight-loss trials, affecting up to 1 in 10. It is often linked to eating and drinking much less than before, especially in the first weeks and after a dose increase.

Eating regular small meals rather than skipping them, and drinking enough fluid through the day, can help. Dizziness and low blood pressure are also listed side effects, so if you feel faint or light-headed when you stand up, sit or lie down until it passes, and don't drive until you feel well. Tell us if tiredness doesn't improve or stops you doing your normal day.

Hair loss

Hair loss is a common side effect, affecting up to 1 in 10 people. In the Mounjaro weight-loss trials it was reported by 4.9% of people on tirzepatide and 1.0% on placebo; it was mostly mild, and most people recovered while staying on treatment. In the Wegovy trials it was reported by 2.5% on the 2.4 mg injection and 1.0% on placebo, and by 6.4% on the Wegovy Pill and 2.0% on placebo.

Hair shedding is also known to follow fast weight loss in general. If you notice it, tell your prescriber: they can check whether the pace of weight loss or your diet needs looking at.

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.
• Sudden loss of vision or worsening eyesight on Wegovy or the Wegovy Pill. A very rare eye condition (NAION) is listed in the leaflet. Contact a doctor urgently.

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.

The effect of these medicines on fertility in people, men or women, is not known. In animal studies, tirzepatide showed no direct harmful effect on fertility, and semaglutide did not affect male fertility in rats. If you or your partner are trying for a baby, tell the prescriber.

Alcohol

The NHS advises that it's best to avoid alcohol while using tirzepatide (Mounjaro), because it can make side effects such as feeling or being sick worse. There is no specific interaction between these medicines and alcohol, but alcohol also adds calories and raises the risk of low blood sugar if you take diabetes medicines. Many people find they want less of it. If you do 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.

Low blood sugar (hypoglycaemia)

Low blood sugar is uncommon on Mounjaro or Wegovy in people who don't have diabetes. The risk is higher if you have type 2 diabetes and also take insulin or a sulfonylurea such as gliclazide. In that case the SmPCs say the dose of those medicines may need to come down, and you may be asked to check your blood sugar more often while your dose goes up.

Signs of low blood sugar include feeling shaky, sweaty, dizzy, weak or confused, a fast heartbeat, headache or blurred vision. If you have these, eat or drink something sugary straight away and tell your diabetes team or prescriber.

Insulin should only be reduced step by step, under your diabetes team's guidance. The MHRA has warned of diabetic ketoacidosis, a dangerous build-up of acids in the blood, when insulin was cut back quickly or stopped after starting a GLP-1 medicine.

Mood, depression and suicidal thoughts

Low mood, depression and suicidal thoughts are not listed as side effects in the UK product information for Mounjaro, Wegovy or the Wegovy Pill. After a full safety review, the MHRA concluded that the available data do not support a causal link between GLP-1 medicines and depression, suicidal thoughts or suicide. It continues to monitor this closely.

Big changes in eating and weight can still affect how you feel. If your mood drops, or you have thoughts of harming yourself, tell your prescriber or GP. If you feel you might act on those thoughts, call 999 or go to A&E, or call Samaritans free on 116 123.

Long-term safety

These medicines are newer than many, so the evidence is still growing. The largest Wegovy study, SELECT, followed about 8,800 people on semaglutide for a median of just over three years. The MHRA keeps all GLP-1 medicines under review and has strengthened warnings over time, for example on pancreatitis.

Gallstones are more likely the more weight you lose, so they matter more as treatment goes on. In rat studies, both medicines caused thyroid tumours of a type called C-cell tumours. For tirzepatide the SmPC says the relevance to people is unknown; for semaglutide it says the relevance is considered low but can't be ruled out.

Treatment is reviewed as you go. If you haven't lost at least 5% of your starting weight after six months (for Mounjaro, six months on your highest tolerated dose), the licence says your prescriber should consider whether to continue.

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.