Short answer: Mounjaro, Wegovy injections, Wegovy tablets and Foundayo share the same main side effects: nausea, vomiting, diarrhoea, constipation and stomach pain, each listed as very common (more than 1 in 10 people) in their UK product information (SmPC, section 4.8). They are mostly mild to moderate and most frequent while the dose is going up. Serious side effects are less common: pancreatitis is uncommon (up to 1 in 100; the MHRA strengthened its warning in January 2026), gallstones are common, dehydration can affect the kidneys, and with semaglutide, the medicine in Wegovy, a sudden loss of vision called NAION is very rare (up to 1 in 10,000). The MHRA has licensed all four for adults they suit, but they are not for everyone, and pens bought outside a registered pharmacy may be fake.
This guide sets the four GLP-1 weight loss medicines we supply side by side, from the UK product information and the MHRA, as of October 2026: how common each side effect is, how long they last, the serious ones and their signs, who should not take these medicines, and how to report a side effect. Each medicine has its own guide, linked below, for the detail.
Are weight loss injections safe?
For most adults they are licensed for, yes, with known side effects. The MHRA has licensed Mounjaro, Wegovy, Wegovy tablets and Foundayo for weight management in adults with a BMI of 30 or above, or 27 or above with a weight-related condition, and explains that licensed means assessed carefully and approved as safe and effective for certain uses. It has not assessed them outside that use, such as losing weight for cosmetic reasons, however often the media call them "skinny jabs". The NHS says most adults can use semaglutide and tirzepatide, but not everyone: none should be used in pregnancy, and some health conditions, such as a past inflamed pancreas, need checking first. So your health and medicines need checking first: through Solira, a GPhC-registered pharmacist reviews every consultation before anything is supplied. Not sure of your BMI? Use our BMI calculator.
What are the most common side effects of weight loss injections and tablets?
The most common side effects of all four are stomach and bowel symptoms: nausea, vomiting, diarrhoea, constipation and stomach pain are very common with each. The table gives each medicine's own frequency band from its UK product information.
| Side effect | Mounjaro | Wegovy injection | Wegovy tablets | Foundayo |
|---|---|---|---|---|
| Nausea | Very common | Very common | Very common | Very common |
| Vomiting | Very common | Very common | Very common | Very common |
| Diarrhoea | Very common | Very common | Very common | Very common |
| Constipation | Very common | Very common | Very common | Very common |
| Stomach pain | Very common | Very common | Very common | Very common |
| Indigestion | Common | Common | Very common | Very common |
| Headache | Not listed | Very common | Very common | Common |
| Tiredness | Common | Very common | Very common | Common |
| Dizziness | Common | Common | Common | Common |
| Reflux or heartburn | Common | Common | Common | Common |
| Bloating, burping and wind | Common | Common | Common | Common |
| Hair loss | Common | Common | Common | Common |
| Gallstones | Common | Common | Common | Common |
| Redness or itching where you inject | Common | Common | Does not apply | Does not apply |
| Low blood pressure | Common | Uncommon | Uncommon | Common |
| Pancreatitis | Uncommon | Uncommon | Uncommon | Uncommon |
| Sudden loss of vision (NAION) | Not listed | Very rare | Very rare | Reported with similar medicines, no frequency given |
Very common means more than 1 in 10 people, common up to 1 in 10, uncommon up to 1 in 100 and very rare up to 1 in 10,000. Not listed means it is not in that medicine's table of side effects. Sources: the UK Summaries of Product Characteristics, section 4.8, on the electronic medicines compendium: Mounjaro KwikPen (emc 15481), Wegovy FlexTouch (emc 13803), Wegovy tablets (emc 102344) and Foundayo (emc 102527). Mounjaro's bands are those of its weight management trials, where its product information gives them. These were separate trials in different groups of people, so the figures are not a direct comparison. Individual results vary.
Mounjaro
Mounjaro's product information also lists a smaller appetite as very common. Headache and NAION are not listed. Hair loss is common, and most people in the trials recovered while staying on treatment. More in Mounjaro side effects, constipation on Mounjaro, tiredness on Mounjaro and hair loss on Mounjaro.
Wegovy injections
Gastritis (an inflamed stomach lining), a changed sense of taste and altered skin sensation are also common, and so is low blood sugar in people with type 2 diabetes. A smaller appetite is not listed as a side effect. Altered skin sensation and hair loss were reported more often at the 7.2mg dose. See Wegovy side effects, tiredness on Wegovy, hair loss on Wegovy and sulphur burps on Wegovy.
Wegovy tablets
The tablets contain the same medicine as the injections, semaglutide, and their product information says their safety in the main trial was consistent with the injection's, except that indigestion was more frequent. See Wegovy tablets side effects.
Foundayo
Foundayo (orforglipron) is a once-daily GLP-1 tablet. A fast heartbeat and low blood pressure are common, and low blood pressure happened more often in people also taking blood pressure medicines. Its trials used a capsule form of the medicine. See Foundayo side effects.
How long do the side effects of weight loss injections last?
For most people, the stomach side effects are worst in the first weeks and after each dose increase, then ease. The product information for all four says nausea, vomiting and diarrhoea were most frequent while the dose was going up, and most were short-lived or became less frequent over time.
- The first weeks
Stomach side effects are most likely as you start. Each of the four starts on a low dose and goes up in steps, which the Wegovy product information says is to make them less likely.
- After each step up
They are more likely again after a dose increase, the MHRA says, and can last for several days.
- Up to about week 20
In the 176-week SURMOUNT-1 results for Mounjaro, and in the STEP UP trials of the higher Wegovy dose, stomach side effects happened mainly in the first 20 weeks, while the dose was going up.
- On a steady dose
Nausea, vomiting and diarrhoea become less frequent over time. Constipation can last longer, the Wegovy product information notes.
Hair loss is usually temporary too: most people in the Mounjaro and Wegovy injection trials recovered while staying on treatment, and half of those affected in the main Wegovy tablets trial had recovered by its end. If a side effect is not settling, the dose does not have to keep rising: the Wegovy product information, for example, says to consider delaying an increase or going back to the previous dose until stomach symptoms improve. The NHS advises speaking to a pharmacist about side effects that bother you or do not go away, and your pharmacist reviews you before each dose increase.
What are the serious side effects of weight loss injections?
The serious ones to know are pancreatitis, gallbladder problems, dehydration that affects the kidneys, low blood sugar alongside insulin or a sulphonylurea, a sudden loss of vision called NAION with semaglutide, and food from the stomach reaching the lungs under anaesthesia. All are much less common than the stomach symptoms.
| Risk | How common | Signs to look out for |
|---|---|---|
| Pancreatitis (an inflamed pancreas) | Uncommon with all four, up to 1 in 100. Rare severe and fatal cases have been reported | Severe stomach pain that does not go away and may spread to your back, with or without vomiting |
| Gallstones and gallbladder problems | Gallstones are common with all four. Mounjaro's and Foundayo's product information link gallbladder problems to weight loss | Pain in the upper right of your tummy, fever, or yellowing of the skin or eyes |
| Dehydration, which can affect the kidneys | No frequency given. Vomiting and diarrhoea, the usual cause, are very common | Dizziness, passing much less urine, a dry mouth |
| Low blood sugar | Mainly with insulin or a sulphonylurea in type 2 diabetes, when it can be very common | Sweating, shaking, sudden hunger, confusion |
| NAION, a sudden loss of vision from the optic nerve | Very rare with semaglutide (Wegovy injections and tablets), up to 1 in 10,000. Not listed for Mounjaro. Reported with similar medicines in Foundayo's product information | Sudden, usually painless, loss of all or part of the vision, usually in one eye |
| Food or drink reaching the lungs during anaesthesia | Reported with these medicines; no frequency given | Tell your surgical or anaesthetic team before any operation or sedation |
| Severe allergic reaction | Rare with Mounjaro and Wegovy. Reported with similar medicines in Foundayo's product information | Swelling of the face, lips or throat, difficulty breathing, or a widespread rash. Call 999 |
In January 2026 the MHRA strengthened the pancreatitis warnings for GLP-1 medicines, Mounjaro and Wegovy included, after rare reports of severe (necrotising) and fatal cases. It had received 1,296 UK Yellow Card reports of pancreatitis with these medicines from 2007 to October 2025, 19 of them fatal, against an estimated 25.4 million packs dispensed in five years. If pancreatitis is confirmed, the medicine should not be restarted. In February 2026 it warned that semaglutide is very rarely linked to NAION, and said it is reviewing the evidence for other GLP-1 medicines.
When to get help: our pharmacist's advice
Stop your treatment and seek urgent medical help if you experience: severe, persistent stomach pain, especially spreading to your back, with or without vomiting. This can be a sign of pancreatitis. Sudden pain in the upper right of your tummy, fever, or yellowing of the skin or eyes: possible gallbladder problems. Sudden loss of vision, or loss of part of your field of vision: seek same-day eye assessment. Swelling of the face or lips, difficulty breathing, or a widespread rash: call 999.
Before any operation: tell your surgical or anaesthetic team you take a GLP-1 medicine. It slows stomach emptying, which matters for anaesthesia.
Keep taking the medicine you already have as usual, unless your surgical or anaesthetic team tells you otherwise.
Which weight loss injection has the fewest side effects?
None has the fewest for everyone, and there is no single safest one: Mounjaro and Wegovy belong to the same family of medicines, share the same main side effects and warnings, and were tested in separate trials in different groups of people, so their rates cannot be compared directly. We do not rank them. The product information does list a few differences, as the table shows: headache is very common with Wegovy and not listed for Mounjaro, low blood pressure is common with Mounjaro and uncommon with Wegovy, and NAION is listed for semaglutide but not for Mounjaro. Which suits you depends on your health, your other medicines and how you got on with any earlier treatment, and a GPhC-registered pharmacist reviews that. See Mounjaro vs Wegovy.
Are weight loss tablets safer than injections?
Not as a rule: Wegovy tablets contain the same medicine as Wegovy injections and carry the same warnings, and Foundayo's product information lists the same family of stomach side effects and the same serious risks, such as pancreatitis. What changes is how you take them.
- There is no injection, so no redness, itching or soreness where you inject.
- Wegovy tablets are taken on an empty stomach, with a wait before food or drink, and indigestion is very common with them. See how to take the Wegovy pill.
- Foundayo is taken at any time of day, with or without food, but some other medicines, such as the antibiotic clarithromycin, limit the highest dose. See how to take Foundayo.
A tablet or an injection is a question of what suits you, not of which is safer. See the Wegovy pill vs the injection.
Are there long-term side effects of GLP-1 medicines?
No new safety concerns have appeared in the longest trials so far, but these medicines are still closely monitored: all four carry the black triangle (▼) for additional monitoring.
- SURMOUNT-1, 176 weeks. In the longest weight management results for Mounjaro (New England Journal of Medicine, 2025), 1,032 adults with obesity and prediabetes took tirzepatide or placebo. Stomach side effects were mostly mild to moderate and mainly in the first 20 weeks, and no new safety signals were identified.
- SELECT, an average of 39.8 months. This trial (New England Journal of Medicine, 2023) followed 17,604 adults with established cardiovascular disease and a BMI of 27 or above, without diabetes, on Wegovy 2.4mg or placebo. Adverse events (any health problem during the trial, caused by the medicine or not) led 16.6% on Wegovy to stop, against 8.2% on placebo. Individual results vary.
- Thyroid. Tirzepatide and semaglutide caused thyroid C-cell tumours in 2-year rodent studies. The product information calls the relevance for people unknown for Mounjaro, and low but not completely excluded for Wegovy. Orforglipron, in Foundayo, does not act in rats or mice, so Foundayo's product information draws on other GLP-1 medicines and reaches the same view as Wegovy's. We do not supply these medicines if you or a blood relative have had medullary thyroid cancer or MEN-2.
More in staying on Mounjaro long term and Is Wegovy safe?
Who should not take a weight loss injection?
The UK product information for all four gives one contraindication, an allergy to the medicine or any of its ingredients, and says none should be used in pregnancy. Our consultation also asks about the conditions below. If one applies, we do not supply weight loss treatment through our service, and your GP is the right person to talk to:
- Cirrhosis or severe liver disease (liver failure, or told you are Child-Pugh C)
- Chronic kidney disease (eGFR < 30 mL/min)
- Heart failure that leaves you breathless at rest or unable to do any activity (NYHA class 4)
- History of pancreatitis
- Medullary thyroid cancer (MTC) or MEN-2, in you or a blood relative
- Currently having chemotherapy, immunotherapy or other active cancer treatment
- Type 1 diabetes
- An eating disorder diagnosed by a doctor, now or in the past (anorexia, bulimia, binge eating disorder)
- Gallbladder problems right now: gallstones causing pain, cholecystitis, or waiting for gallbladder surgery
- Gastroparesis (delayed stomach emptying) or a previous bowel obstruction
We also do not supply these medicines if you are pregnant, breastfeeding or trying for a baby, or if you take insulin or a sulphonylurea (such as gliclazide). GLP-1 treatment with insulin or a sulphonylurea can cause dangerously low blood sugar (hypoglycaemia). This needs to be managed by your GP or diabetes team, who can adjust your existing medicines at the same time. See medicines you cannot take with Mounjaro or Wegovy, contraception and pregnancy on GLP-1 treatment and Is Mounjaro safe?
Depression is not listed as a side effect of any of the four, and the MHRA says the available data does not support a causal link between GLP-1 medicines and depression. Tell your pharmacist about any mental health condition. See whether Mounjaro can cause depression.
Wondering which of these medicines might suit you? Our consultation asks about your health and the medicines you take, and a GPhC-registered pharmacist reviews every answer before anything is supplied.
Check your eligibilityHow do you report a side effect?
Report it to the MHRA's Yellow Card scheme, on the Yellow Card website or app, and tell your pharmacist too.
Tell your pharmacist. Message us through your account, so your pharmacist can review your treatment.
Report it on Yellow Card. You do not need to be sure the medicine caused it: a suspicion is enough.
Give the details. The MHRA asks for the medicine and its batch number, when the side effect started, your treatment dates and your other medicines.
Keep a pen or pack you are unsure of. Yellow Card also takes reports of medicines you think may not be genuine.
The black triangle (▼) on all four means they are under additional monitoring, so new safety information can be identified quickly. The MHRA says it does not mean a medicine is unsafe.
Why are fake weight loss injections dangerous?
Because a fake may not contain what its label says: the MHRA has had reports of severe side effects from fake GLP-1 medicines, and some have been found to contain insulin. Genuine GLP-1 medicines come as pre-filled pens or tablets; a powder in a vial to mix yourself is never an authorised product, and counterfeit pens can closely copy real ones. Selling these medicines through salons or social media is illegal, the MHRA says. Any pharmacy in Great Britain can be checked on the General Pharmaceutical Council's register: QuickCare Pharmacy, the registered pharmacy that dispenses Solira orders, is GPhC 9013213. How to protect yourself is in Is Mounjaro safe? and on our safety page.