Every weight loss pen has side effects, and most people want the honest version: how often each one happens, which are serious, and who the pens are not for. This page sets out the real frequencies for Mounjaro and Wegovy from the UK labels and the published trials, explains the interaction with insulin and sulfonylureas, and shows how to report a problem. Every number is drawn from a source listed at the end.
Weight loss pen side effects are mostly gastrointestinal — nausea, diarrhoea, vomiting and constipation — usually mild to moderate, and worst while the dose is being increased. On the UK label, nausea affects around 44% of Wegovy users and, in the Mounjaro trial, roughly a quarter to a third of people. Serious effects such as pancreatitis, dehydration and gallbladder problems are less common but real. A prescriber, not a website, weighs these risks for you.
Why the side effects are mostly in the gut
To understand the side effects it helps to know how a weight loss pen works. Both medicines mimic natural gut hormones: they prompt insulin release when blood sugar rises, reduce appetite, and slow how quickly the stomach empties.[1][2] That last effect — slowed gastric emptying — is precisely why the most common side effects are digestive.
The pattern across the class is consistent: the effects are predominantly gastrointestinal, mostly mild to moderate, dose-related, and most common during dose escalation, settling over the following weeks.[3] That is why the dose is raised slowly — Mounjaro starts at a non-therapeutic 2.5 mg once weekly for four weeks, and Wegovy climbs over 16 weeks from 0.25 mg to the 2.4 mg maintenance dose.[1][2] The step-up is not optional pacing; it reduces the reactions that peak when a dose first goes up. Our using your pen guide covers that early period.
Wegovy (semaglutide): the frequencies from the UK label
The UK Summary of Product Characteristics reports pooled frequencies from the semaglutide trial programme. The four dominant effects are all digestive and all "very common" — meaning they affect at least 1 in 10 people.[1]
| Side effect | Frequency |
|---|---|
| Nausea | 43.9% |
| Diarrhoea | 29.7% |
| Vomiting | 24.5% |
| Constipation | 24.2% |
Beyond those four, the label lists a range of common effects (up to 1 in 10 people): headache, dizziness, abdominal pain, gastritis, reflux, indigestion, burping, wind, bloating, delayed gastric emptying and gallstones. Two that often surprise people also appear here — hair loss (about 2.5% of users) and a small average rise in heart rate of around 3 beats per minute.[1] The Wegovy pen page profiles the device and dosing.
Mounjaro (tirzepatide): the frequencies from the trial
The Mounjaro UK label lists nausea, diarrhoea, vomiting, constipation and abdominal pain as very common (at least 1 in 10 people).[2] For actual percentages, the clearest source is SURMOUNT-1, the 72-week weight-management trial, which reported the rates at each dose against placebo.[3]
| Side effect | 5 mg | 10 mg | 15 mg | Placebo |
|---|---|---|---|---|
| Nausea | 24.6% | 33.3% | 31.0% | 9.5% |
| Diarrhoea | 18.7% | 21.2% | 23.0% | 7.3% |
| Constipation | 16.8% | 17.1% | 11.7% | 5.8% |
| Vomiting | 8.3% | 10.7% | 12.2% | 1.7% |
Two cautions on those numbers. The placebo column is not zero — about one person in ten reported nausea even on a dummy injection — so the medicine's true added effect is the gap between the columns, not the raw percentage. And the Mounjaro figures come from a single trial at fixed doses while the Wegovy figures above are pooled across a whole programme, so the two tables are not a like-for-like head-to-head.
The Mounjaro label also lists common effects (up to 1 in 10): indigestion, bloating, burping, wind, reflux, gallstones, decreased appetite, dizziness, tiredness, injection-site reactions, a faster heart rate, raised pancreatic enzymes (lipase and amylase), hair loss, and low blood sugar when combined with other diabetes medicines.[2] Gallstones (cholelithiasis) were reported in roughly 0.3% of the diabetes trials and about 1.1% in the weight-management setting.[2] The Mounjaro pen page covers the device and dosing.
What matters as much as who felt sick is whether the effects made people stop. In SURMOUNT-1, discontinuation because of adverse events was 4.3% at 5 mg, 7.1% at 10 mg and 6.2% at 15 mg, against 2.6% on placebo.[3] So even at the higher doses, well under one in ten stopped specifically because of side effects — not nothing, but consistent with the "mostly mild to moderate" picture.
The serious but less common risks
These events are uncommon, but they are the ones to recognise early — and the reason weight loss pens are prescription-only rather than off-the-shelf.
- Acute pancreatitis. Sudden, severe inflammation of the pancreas has been reported with this class; the label's advice is to stop the medicine and seek help if it is suspected — typically severe, persistent stomach pain.[2]
- Dehydration and kidney injury. Nausea, vomiting and diarrhoea can cause dehydration and, rarely, a deterioration in kidney function including acute kidney injury — which is why persistent symptoms should be reported rather than simply endured.[1]
- Gallbladder problems. Gallbladder problems, including gallstones (cholelithiasis), are recognised adverse events with these medicines and are listed as a common effect on both labels.[1][2]
- Diabetic retinopathy. An increased risk of diabetic retinopathy complications has been observed with semaglutide; this is relevant chiefly to people who also have diabetes.[1]
Get urgent medical advice for severe, persistent stomach pain (with or without vomiting), which can signal pancreatitis or gallbladder disease, and for vomiting or diarrhoea you cannot keep under control, because dehydration can injure the kidneys.[1][2] None of this replaces the advice of your GP or pharmacist, who can see your history and the current label.
The insulin and sulfonylurea interaction: low blood sugar
On their own, weight loss pens carry a low risk of hypoglycaemia (low blood sugar), because they raise insulin only when blood sugar is high. The picture changes when a pen is combined with insulin or a sulfonylurea — two treatments used mainly for type 2 diabetes. The UK product information states there is an increased risk of hypoglycaemia in that combination, and that a dose reduction of the insulin or the sulfonylurea may be needed.[2]
So if you use insulin or a sulfonylurea (names often ending in "-ide", such as gliclazide), this is a conversation to have with your prescriber before starting, not after — and it is worth knowing the signs of low blood sugar, such as shakiness, sweating, hunger and confusion.
Who these pens are not for
The formal contraindication on the UK labels is short. For both Mounjaro and Wegovy, the only listed contraindication is a known allergy (hypersensitivity) to the active medicine or to any of the pen's other ingredients.[1][2] Beyond that, neither pen should be used in pregnancy.[1]
There is one interaction worth flagging for anyone who could become pregnant. The manufacturer's UK product information for Mounjaro advises that, because the medicine can delay gastric emptying, reduced effectiveness of oral contraceptives (the pill) cannot be excluded in women with obesity; it recommends adding a barrier method, such as condoms, around starting and around each dose increase.[2] And the MHRA has stated it has not assessed the safety or effectiveness of these medicines outside their licensed use — for example semaglutide for people who are not overweight or obese — so off-label use goes without that safety assessment.[5]
The US thyroid warning is not the UK label
You may read, especially on US sources, that these medicines carry a "black box" thyroid cancer warning. That is a real feature of the US prescribing information: it carries a boxed thyroid C-cell (medullary) tumour warning, based on findings in rodents, and contraindicates a personal or family history of medullary thyroid carcinoma and of the syndrome MEN 2.[4] The UK and EU labels do not list those as contraindications — they treat thyroid risk as a precaution rather than a boxed warning.[2] So the US boxed warning is not the UK position; if you have a thyroid history, raise it with your prescriber either way.
Reporting side effects: the Yellow Card scheme
If you experience a suspected side effect from any medicine — a weight loss pen or anything else — report it through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk, and speak to your GP or pharmacist.[6] Yellow Card reports are how the UK regulator spots safety signals that individual trials — being relatively small and short — are not designed to catch. You do not need to be certain the medicine was the cause; a suspicion is enough, and patients and carers can report, not only clinicians.
Side-effect risk multiplies if the pen is not a genuine, regulated medicine. Weight loss pens should only ever be obtained on prescription through a GPhC-registered pharmacy. Buying weight loss injections from an unregulated seller — a social-media account, or a site with no prescriber — is a real danger, because what arrives may not be what the label claims. The MHRA's FakeMeds campaign explains how to check.[7] We do not sell, supply or link to any provider.
Frequently asked questions
What are the most common side effects of weight loss pens?
Gastrointestinal ones. On the UK label, Wegovy's most common effects are nausea (43.9%), diarrhoea (29.7%), vomiting (24.5%) and constipation (24.2%).[1] Mounjaro's label lists the same family as very common; in SURMOUNT-1 nausea affected roughly a quarter to a third of people across doses.[2][3] Most are mild to moderate and worst while the dose is going up.
Can weight loss pens cause low blood sugar?
On their own the risk is low, but it rises when the pen is combined with insulin or a sulfonylurea; the UK product information notes the dose of those medicines may need to be reduced.[2] Anyone using them should be reviewed by their prescriber and know the signs of low blood sugar — shakiness, sweating and confusion.
Do weight loss pens carry a thyroid cancer warning in the UK?
Not in the way the US label does. The US prescribing information carries a boxed thyroid C-cell tumour warning and contraindicates a personal or family history of medullary thyroid carcinoma and MEN 2.[4] The UK and EU labels do not list those as contraindications and treat thyroid risk as a precaution; the only UK contraindication is a known allergy to the medicine or the pen's ingredients.[2]
How do I report a side effect from a weight loss pen?
Report suspected side effects from any medicine through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk, and speak to your GP or pharmacist.[6] A suspicion is enough. Our FAQ answers more general questions about the pens.
References
- Electronic Medicines Compendium (emc). "Wegovy (semaglutide) FlexTouch pre-filled pen — Summary of Product Characteristics" (product 13803): side-effect frequencies, common effects, dehydration and renal warning, diabetic retinopathy, pregnancy and contraindications. medicines.org.uk
- Electronic Medicines Compendium (emc). "Mounjaro (tirzepatide) KwikPen / pre-filled pen — Summary of Product Characteristics" (product 15481): very common and common side effects, gallstones, pancreatitis, hypoglycaemia with insulin/sulfonylurea, oral contraceptive advice and contraindications. medicines.org.uk
- Jastreboff AM et al. "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1). New England Journal of Medicine, 2022 — per-dose gastrointestinal adverse-event frequencies, treatment discontinuation rates, and the mostly mild-to-moderate, dose-related pattern. nejm.org
- US Food and Drug Administration. "Mounjaro (tirzepatide) US Prescribing Information" (NDA 215866, 2022) — boxed thyroid C-cell tumour warning and US contraindications (cited for the US-versus-UK contrast only). accessdata.fda.gov
- MHRA / GOV.UK. "MHRA updates guidance for semaglutide prescribers and patients" — the MHRA has not assessed safety or effectiveness outside licensed use. gov.uk
- MHRA. "Yellow Card scheme — report a suspected side effect." yellowcard.mhra.gov.uk
- MHRA / GOV.UK. "FakeMeds — buying medicines safely online." fakemeds.campaign.gov.uk