Feeling sick on Mounjaro or Wegovy: when to hold your dose

Feeling sick is the most common side effect of Mounjaro and Wegovy. It's most likely in the weeks after starting or going up a dose, it's usually mild to moderate, and it tends to ease with time. ^{[1] [2]}

In the trials behind their UK labels, between 1 in 4 and 3 in 10 people on Mounjaro felt sick, depending on the dose, and more than 4 in 10 on Wegovy 2.4mg. Some people on a dummy injection felt sick too: about 1 in 12 in the Mounjaro trials and 1 in 6 in the Wegovy ones. Fewer than 1 in 20 people taking either medicine stopped because of stomach side effects. ^{[1] [2]}

The usual advice is the same everywhere: smaller meals, less fat, more water. But most articles skip other questions that matter once you're actually being sick. Does feeling rough mean it's working? Do anti-sickness tablets help? Do you have to go up a dose when you're struggling? And what do you do about your injection after a night of being sick?

The British Dietetic Association's guide for people taking these medicines, written by dietitians, suggests the following. ^[3]

• Have three small meals a day and avoid long gaps between them. If a meal looks too big, eat the protein first.
• Eat and chew slowly.
• If spicy or high-fat food makes you feel worse, have less of it or swap it for something else.
• Limit or avoid alcohol, sugary food and drinks, and fizzy drinks like cola or lemonade. The NHS also warns that alcohol can make sickness worse. ^[4]
• Keep drinking through the day, aiming for 1.5 to 2 litres. Water and caffeine-free or low-sugar drinks are best, and if you're not very hungry, drink between meals rather than with them.

The guide was part-funded by Novo Nordisk and Eli Lilly, which make Wegovy and Mounjaro. The BDA says neither had any input into its content. ^[3]

Try not to let feeling sick turn into barely eating. The same guide warns that eating too little can leave you dizzy, tired and low, and can cause hair loss, which we cover in our guide to Mounjaro, Wegovy and hair loss. ^[3] If your problem is constipation rather than sickness, our constipation guide explains which laxative suits which problem.

No, although it's easy to see why people think so. If you feel sick, you eat less.

Wegovy's summary of product characteristics (SmPC) says the weight loss in its trials happened whether or not people had nausea, vomiting or diarrhoea. ^[2] A pooled analysis of three of those trials found people lost a similar amount of weight with and without stomach side effects, and concluded the weight loss was largely independent of them. ^[5] An analysis of the main Mounjaro trial, reported by its manufacturer Eli Lilly, found the same: average weight loss was consistent whether or not people had stomach side effects. ^[6]

Both analyses used the manufacturers' own trial data, but they point the same way. These medicines work by making you feel fuller and less hungry, not by making you ill. ^{[2] [4]} Feeling worse doesn't mean it's working better, and feeling fine doesn't mean it isn't working.

Prochlorperazine acts on the part of the brain that triggers sickness and starts working in around 30 to 60 minutes. ^{[14] [18]} It also comes as a tablet that dissolves between your upper gum and lip, so you don't have to swallow it. ^[14] It can make you sleepy, and its SmPC warns it can lower your blood pressure, usually when you stand up, particularly if you're short of fluids. Mounjaro and Wegovy can lower blood pressure too. ^{[1] [2] [14] [18]} Rarely, it causes muscle stiffness, shaking or uncontrollable movements of the face or tongue, which need a call to your doctor or 111 straight away. ^[19] For sickness like this it needs a prescription, as the versions sold in pharmacies are only licensed for sickness with migraine. ^[18]

Cyclizine is one of the anti-sickness medicines we're asked for most by people on these injections and is the most common medication to take to treat nausea/sickness associated with GLP-1s. It's an antihistamine that starts working in about 30 minutes and lasts 4 to 6 hours. ^[13] It can make you drowsy and give you a dry mouth or blurred vision, and its SmPC also lists constipation and reflux, which these injections already cause. ^{[1] [2] [13] [17]} It isn't suitable for everyone, including people with glaucoma, epilepsy, heart failure, liver problems or trouble peeing. ^[17]

There are no direct studies that look at these medications for treating GLP-1 induced nausea/sickness. The closest thing we have is a 2010 study in 120 healthy volunteers given a single dose of an older GLP-1 medicine, exenatide. Those who took two other anti-sickness medicines beforehand, metoclopramide and ondansetron, were much less likely to feel sick: 17% against 62%. But it wasn't randomised, and its authors worked for exenatide's makers. ^[15]

No. The gradual increases are there to keep side effects down while your body gets used to the medicine. They're not a timetable you have to hit or you won't lose weight. ^[2]

Wegovy's SmPC says that if stomach side effects are significant, your prescriber should consider delaying the next increase or taking you back to the previous dose until things settle. ^[2] Mounjaro's treats every increase after the first as 'if needed', with at least 4 weeks on each dose, and lists 5mg, 10mg and 15mg as doses people can stay on long term. ^[1]

Even in the head-to-head trial of the two medicines, people who couldn't cope with the top dose were moved down a step. ^[1]

If you're still struggling when your next increase is due, tell your prescriber before you go up. Don't change your dose on your own.

Being sick or having diarrhoea once is unlikely to dehydrate you, but if it happens more than once you could be at risk of dehydration.^[7]

Sickness and diarrhoea on these medicines can lead to dehydration, which can affect your kidneys. ^{[1] [2]} The MHRA, the UK's medicines regulator, says it sometimes becomes serious enough to need hospital treatment. ^[8]

Keep your fluids up

Sip fluids little and often, and avoid alcohol and caffeine while you're unwell, as both dehydrate you. If you're losing a lot of fluid, a pharmacist can recommend an oral rehydration solution to put back the salts and sugar you've lost. ^[9]

Should you take your injection?

Not while you're being sick. Sick day guidance from Gloucestershire Hospitals NHS Foundation Trust lists Wegovy and Mounjaro among the medicines to stop while you're losing fluids through vomiting or diarrhoea, and to restart once you've been eating and drinking normally for 24 to 48 hours. ^[7] The Northern Ireland formulary includes these medicines in its sick day rules too. ^[10]

Once you've been eating and drinking normally for a day or two, check how long it's been since your injection was due.

• Mounjaro: if it's 4 days or less, take it then. If it's more than 4 days, skip it and take your next one on your usual day. ^{[1] [4]}
• Wegovy: the same, but the cut-off is 5 days. ^[2]
• More than one dose missed: speak to your prescriber before restarting. Wegovy's label says a lower restart dose should be considered. ^[2]

The same sick day lists include anti-inflammatory painkillers like ibuprofen and naproxen, and some blood pressure and water tablets. ^{[7] [10]} If you take any of those, ask a pharmacist which to pause too. Our naproxen guide explains why that mix is hard on the kidneys.

Get urgent medical help for severe stomach pain that doesn't go away, especially if it spreads to your back. It can be a sign of an inflamed pancreas, which is rare but can be serious. ^{[4] [8]}

Call 999 or go to A&E if you have signs of dehydration and you're confused, much sleepier than normal or hard to wake, have cold, pale or blotchy skin, or are struggling to breathe. ^[9]

Call NHS 111 or ask for an urgent GP appointment if you feel unusually tired, feel dizzy when you stand up and it doesn't go away, have dark yellow pee or are peeing less, or have a fast heartbeat or are breathing quickly. ^[9] Call 111 too if you think you're having another serious side effect, such as severe tummy pain from gallstones.

Speak to your prescriber if the sickness bothers you or isn't settling, if you're finding it hard to eat, or before your next dose increase if you're still struggling. ^{[3] [4]}