Feeling a bit queasy after starting a new weight loss medication is common, and for most people it settles down within a few weeks.
But “common and manageable” isn’t the same as “ignore it and carry on”. A small number of side effects, across every weight loss medicine covered here, are signs that something needs urgent medical attention.
This guide walks through what’s typically expected with Mounjaro, Wegovy, Foundayo, Wegovy Tablets and Orlistat, and the specific warning signs that mean you should stop and seek help rather than wait it out.
This article is for general information only and does not replace personalised medical advice. If you’re ever unsure whether a symptom is serious, contact your prescriber, call NHS 111, or in an emergency call 999. Don’t wait to “see if it gets better” first.
Why These Side Effects Happen
Four of the five medicines in this guide, Mounjaro, Wegovy, Foundayo and Wegovy Tablets, belong to the GLP-1 receptor agonist family (Mounjaro is a dual GIP/GLP-1 agonist).
They all work in a similar way: slowing digestion and reducing appetite. That’s also why they share a similar side-effect pattern, mostly gastrointestinal, especially in the first few weeks or after a dose increase.
Orlistat is different. It doesn’t act on appetite hormones at all. It blocks the absorption of dietary fat. This means its side effects are a completely separate category, closely tied to what you actually eat.
The General Rule: Normal vs Needs Attention
As a broad guide across all of these medicines:
- Usually normal and self-manageable: mild nausea, occasional loose stools or constipation, indigestion, bloating, fatigue, mild injection site reactions, taste changes. These are worth mentioning at your next check-in, but don’t usually need urgent action on their own.
- Contact your prescriber or pharmacist promptly: side effects that are severe, don’t improve, or last more than a couple of days, particularly persistent vomiting or diarrhoea, which can lead to dehydration.
- Seek urgent medical help (111, your GP the same day, or A&E) or call 999: severe or sudden symptoms covered in the red flags section below. These aren’t “wait and see” situations.
Red Flags That Apply Across GLP-1 Medicines (Mounjaro, Wegovy, Foundayo, Wegovy Tablets)
These four medicines share several serious, though uncommon or rare, warnings. If you experience any of the following, stop the medication and seek urgent medical help rather than waiting for your next appointment:
- Severe, persistent abdominal pain (with or without nausea/vomiting) a possible sign of acute pancreatitis.
- Signs of a severe allergic reaction – swelling of the face, lips, tongue or throat, difficulty breathing or swallowing, wheezing, or a fast heartbeat. This needs emergency care (call 999).
- Signs of gallbladder problems – pain in the upper right abdomen (sometimes spreading to the back or shoulder), pale or clay-coloured stools, dark urine, or yellowing of the skin or eyes (jaundice).
- Sudden vision loss or rapidly worsening eyesight – contact a healthcare professional urgently.
- Signs of a possible thyroid problem – a lump or swelling in the neck, hoarseness that doesn’t go away, or difficulty swallowing or breathing. This relates to a specific warning about thyroid C-cell tumours seen in animal studies, and is why these medicines aren’t suitable for anyone with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 (MEN2).
- Signs of dehydration or kidney problems following severe or prolonged vomiting/diarrhoea – reduced urination, unusual weakness, dizziness, or confusion.
- Signs of bowel obstruction – severe, persistent bloating or abdominal pain with vomiting and an inability to pass stool or wind.
Mounjaro (Tirzepatide) – Side Effects
Common: nausea, diarrhoea, reduced appetite, vomiting, constipation, indigestion, abdominal discomfort, acid reflux, bloating, and mild injection site reactions. These are most noticeable in the first few weeks or after each dose increase, and typically improve over time.
More serious – seek medical advice:
- Acute pancreatitis. The UK patient information leaflet for Mounjaro was formally updated in late 2025 to reinforce this as an uncommon side effect (affecting up to 1 in 100 people); the advice is to stop the medicine and seek urgent medical help for severe, persistent stomach pain.
- Gallbladder problems, including gallstones or bile duct blockage. Watch for upper abdominal pain, pale stools, dark urine, or jaundice.
- Signs of kidney injury following severe dehydration from vomiting or diarrhoea such as weakness, reduced urination, or confusion.
- Hypoglycaemia (low blood sugar), particularly if Mounjaro is combined with other diabetes medications. Watch for shakiness, sweating, or confusion.
- Signs of a thyroid tumour or severe allergic reaction, as described in the red flags section above.
Wegovy (Semaglutide Injection) – Side Effects
Very common (more than 1 in 10 people): headache, nausea, vomiting, diarrhoea, constipation, stomach pain, and weakness or fatigue.
Common (up to 1 in 10 people): dizziness, indigestion, burping, flatulence, bloating, reflux, heartburn, gallstones, hair loss, injection site reactions, taste changes, altered skin sensation, and (in people with diabetes) low blood sugar or diabetic eye disease complications.
More serious – seek medical advice:
- Acute pancreatitis (uncommon, up to 1 in 100) – severe, persistent abdominal pain.
- Severe allergic reactions (rare, up to 1 in 1,000) – breathing difficulty, facial/throat swelling, wheezing, or a fast heartbeat.
- Sudden vision loss or rapidly worsening eyesight (very rare) – this is a specific, urgent warning for semaglutide, particularly relevant if you have existing diabetic eye disease.
- Very rare risks including hip fractures and bowel obstruction, and warnings around kidney or bladder stones.
Foundayo (Orforglipron) – Side Effects
Very common (more than 1 in 10 people): nausea, vomiting, constipation, diarrhoea, indigestion, and abdominal pain.
Common (up to 1 in 10 people): dizziness, headache, increased heart rate, low blood pressure, bloating, acid reflux, hair thinning, and fatigue.
More serious – seek medical advice: As a GLP-1 receptor agonist, the same class-wide red flags described above apply – particularly the warnings around pancreatitis, severe allergic reaction, and thyroid tumours (Foundayo carries the same contraindication for anyone with a personal or family history of medullary thyroid carcinoma or MEN2). Because Foundayo is newer to the UK market, having only been authorised in August 2026, it’s worth reading the patient information leaflet supplied with your specific prescription closely, and reporting any side effect, expected or not, via the MHRA Yellow Card Scheme.
Wegovy Tablets (Oral Semaglutide) – Side Effects
Common: nausea, diarrhoea, constipation, and vomiting – most noticeable early in treatment or shortly after a dose increase, and generally easing as your body adjusts.
More serious – seek medical advice: Wegovy Tablets contain the same active ingredient (semaglutide) as the Wegovy injection, so the same class warnings apply: acute pancreatitis, severe allergic reactions, sudden vision changes, and the rarer risks of gallbladder problems, kidney or bladder stones, and bowel obstruction described above. One tablet-specific point: if you don’t follow the fasting and water rules correctly (see our guide comparing oral weight management options for the full timing rules), the medicine may simply be absorbed less effectively. This isn’t a safety issue as such, but it’s worth mentioning to your prescriber if you’re struggling with the routine, rather than adjusting the timing yourself.
Orlistat – Side Effects
Because orlistat works by blocking fat absorption rather than affecting appetite hormones, its side effects are a different category altogether and closely linked to how much fat is in your diet.
Common (diet-related): oily or fatty stools, needing to go to the toilet urgently or more often, flatulence (sometimes with oily spotting), stomach pain, and diarrhoea. These typically ease significantly on a lower-fat diet, since they’re a direct result of unabsorbed fat passing through the digestive system.
More serious – seek medical advice:
- Signs of a severe allergic reaction.
- Acute pancreatitis – severe or persistent abdominal or back pain.
- Rectal bleeding.
Any of these three need you to contact NHS 111 or go to A&E promptly, rather than waiting to see if they settle.
One practical note: orlistat can reduce absorption of some fat-soluble vitamins, so a multivitamin supplement (taken at least two hours before or after your dose) is generally recommended. It can also interact with other medicines, including warfarin, thyroid medication, and the contraceptive pill (severe diarrhoea can reduce the pill’s effectiveness). Always tell your prescriber about everything else you take.
Practical Tips for Managing Common Side Effects
- Eat smaller, simpler meals and avoid very fatty, greasy or heavily processed food. This helps with GLP-1-related nausea, and is essential for managing orlistat’s side effects specifically.
- Stay well hydrated, particularly if you’re experiencing any vomiting or diarrhoea, to reduce the risk of dehydration.
- Stand up slowly if you’re prone to dizziness or light-headedness, especially on Foundayo or Wegovy, where low blood pressure has been reported.
- Stick to your prescribed titration schedule rather than increasing your dose faster than advised. Side effects are usually worse when doses go up too quickly.
- Never adjust your own dose or stop treatment abruptly because of a side effect without speaking to your prescriber first, unless you’re experiencing one of the red-flag symptoms above, in which case seek help immediately.
Frequently Asked Questions
Is it normal to feel sick when starting weight loss medication? Yes, mild nausea is one of the most commonly reported side effects across GLP-1 medicines, particularly in the first few weeks or after a dose increase. It usually settles as your body adjusts.
How long do side effects usually last? Most gastrointestinal side effects ease within a few weeks of starting a medicine or increasing a dose. If a side effect is severe, worsening, or hasn’t improved after a couple of weeks, contact your prescriber.
What’s the difference between a normal side effect and a medical emergency? Normal side effects tend to be mild-to-moderate, gradually improve, and don’t involve the red-flag symptoms listed above (severe abdominal pain, allergic reaction signs, sudden vision changes, signs of jaundice, or a neck lump). If in doubt, treat it as something to check rather than something to wait out.
Should I stop my medication if I get side effects? Not usually, for mild and expected side effects but always stop and seek urgent medical help immediately if you experience any of the serious warning signs described above, and never restart without medical advice.
Can I take anything to help with nausea or diarrhoea? Simple dietary changes (smaller meals, avoiding rich or fatty food, staying hydrated) are usually the first step. Speak to your prescriber or pharmacist before taking any over-the-counter remedies, as some can interact with your treatment.
Are the side effects different for tablets versus injections? The underlying side-effect profile is similar within the same drug class (for example, Wegovy Tablets and the Wegovy injection share the same active ingredient and warnings). The main practical difference is the extra fasting and timing rules that come with Wegovy Tablets specifically.
Where do I report a side effect? Any side effect from a UK-licensed medicine, whether expected or not, can be reported to the MHRA via the Yellow Card Scheme, as well as being discussed with your prescriber or pharmacist.
References
- GOV.UK / MHRA – UK first in Europe to authorise orforglipron for weight management and type 2 diabetes
- Wegovy (Novo Nordisk UK) – Safety Information
- The Pharmaceutical Journal – ‘Life-threatening condition’ added to Mounjaro patient information leaflet
- Medical News Today – Mounjaro side effects: Common, serious, and long term
- NHS – Orlistat: a medicine to treat obesity
- Patient.info – Orlistat (Weight Loss Medicine): Uses and Side Effects
- MHRA Yellow Card Scheme – Report a side effect