Updated November 2026 · Based on the UK Foundayo patient leaflet (August 2026) and the ATTAIN-1 trial
The short version: most people who take Foundayo (orforglipron) get some stomach-related side effects, mainly nausea, constipation, diarrhoea or vomiting. They're usually mild to moderate, they're worst in the first weeks and after each dose increase, and for most people they ease over time. About 1 in 10 people on the highest trial dose stopped treatment because of side effects.
The most common side effects
The UK patient leaflet lists these as very common, meaning they affect more than 1 in 10 people:
- nausea (feeling sick)
- constipation
- diarrhoea
- vomiting
- indigestion
- stomach pain
How often they happened in the main trial
In ATTAIN-1, the 72-week trial of 3,127 adults with obesity or overweight, people took one of three doses or a placebo:
| Side effect | Foundayo (range across doses) | Placebo |
|---|---|---|
| Nausea | 29% to 36% | 10% |
| Constipation | 22% to 30% | 9% |
| Diarrhoea | 21% to 23% | 10% |
| Vomiting | 13% to 24% | 4% |
| Indigestion | 13% to 16% | 5% |
Most of these were mild or moderate. Some people on the placebo had the same symptoms, so not every upset stomach is caused by the medicine.
Stopping because of side effects: between about 5% (lowest dose) and 10% (highest dose) of people on Foundayo stopped because of side effects, against about 3% on placebo.
Other common side effects
The leaflet lists these as common (up to 1 in 10 people):
- bloating, burping, wind and heartburn or reflux
- headache, dizziness and tiredness
- a faster pulse and low blood pressure. Tell your prescriber if you take blood pressure tablets.
- hair loss
- gallstones
- raised pancreatic enzymes on a blood test
Hair loss on GLP-1 medicines is usually linked to losing weight quickly and tends to settle. Mention it to your prescriber if it worries you.
If you have type 2 diabetes and take insulin or a sulfonylurea (such as gliclazide), Foundayo raises your risk of low blood sugar. Your diabetes medicines may need adjusting, so tell the prescriber before you start.
When side effects are worst
Side effects follow the dose steps. Foundayo starts at 0.8mg a day and goes up no faster than one step a month (2.5mg, 5.5mg, 9mg, 14.5mg, then 17.2mg). For most people:
- Weeks 1 to 2 on a new dose: nausea and changes in bowel habit are most likely
- Weeks 3 to 4: symptoms usually settle as your body adjusts
- After a dose increase: they can come back for a few days to a couple of weeks
The UK leaflet says your dose should only go up if the current one is working and your side effects are manageable. If they're not, ask your prescriber about staying on your current dose for longer. You don't have to keep moving up.
How to manage the common side effects
Nausea
- eat smaller meals, slowly, and stop as soon as you feel full
- avoid greasy, fried or very rich food, especially after a dose increase
- try plain food (toast, crackers, rice) and sip cold drinks
- Foundayo can be taken at any time, so some people find taking it in the evening helps
Constipation
- drink plenty of water through the day
- add fibre gradually (oats, fruit, vegetables, wholegrains)
- keep moving, as walking helps
- ask a pharmacist about a gentle laxative if it lasts more than a few days
Diarrhoea and vomiting
- sip fluids little and often to avoid dehydration, which can affect your kidneys
- oral rehydration sachets help if it's frequent
- contact your prescriber if you can't keep fluids down
Indigestion, reflux and bloating
- avoid lying down straight after eating
- cut back on fizzy drinks, alcohol and large late meals
Eating enough protein matters too, because it protects your muscle while you lose weight. See our guide to eating on GLP-1 medicines.
The contraceptive pill: an important extra step
The UK leaflet says Foundayo may make oral contraceptives less effective. Use a barrier method such as condoms, or a non-oral method, for 30 days after starting Foundayo and for 30 days after each dose increase. Foundayo shouldn't be used in pregnancy. If you're planning a pregnancy, the leaflet says to stop at least 3 weeks beforehand.
Serious side effects: when to get help
These are rare, but you should know them:
- Pancreatitis (uncommon, up to 1 in 100): severe stomach pain that won't go away and may spread to your back. Stop taking Foundayo and get medical help straight away.
- Bowel obstruction or severe constipation: severe tummy pain and swelling, vomiting, and being unable to pass wind or open your bowels.
- Gallbladder problems: pain in the upper right tummy, fever, yellow skin or eyes, or pale stools.
- Serious allergic reaction: swelling of the face or throat, difficulty breathing or wheezing. Call 999.
- Dehydration and kidney problems: after a lot of vomiting or diarrhoea, especially if you have kidney disease.
- Sudden changes in your vision.
Tell the doctor or anaesthetist if you're having surgery or a procedure under sedation, as GLP-1 medicines can slow how fast your stomach empties.
You can report any side effect through the MHRA Yellow Card scheme. Foundayo carries a black triangle, meaning it's being closely monitored because it's new.
How Foundayo's side effects compare
| Foundayo | Wegovy pill | Mounjaro injection | |
|---|---|---|---|
| Main side effects | Nausea, constipation, diarrhoea, vomiting | Nausea, diarrhoea, vomiting, constipation | Nausea, diarrhoea, constipation, vomiting |
| Injection-site reactions | No | No | Yes, sometimes |
| Food and timing rules | None | Empty stomach, 30-minute wait | None |
| Affects the oral contraceptive pill | Yes, for 30 days after starting and after each increase | Ask your prescriber | Yes, for 4 weeks after starting and after each increase |
All GLP-1 medicines cause similar stomach side effects. The type of medicine matters less than the dose and how quickly it's increased. See Wegovy pill vs Foundayo for a full comparison.
Where to get Foundayo with good support
Clinics that offer regular check-ins and quick messaging make side effects much easier to manage:
- Second Nature: includes a nutritionist and app, from £129 a month
- Boots Online Doctor: pharmacist support and in-store collection, from £99
- Cuva Health: lowest prices, from £94.99 (currently listed as pre-order)
- IQ Doctor: lowest starter price, £90
See every provider's Foundayo price by dose
Keep reading
- How to take the Wegovy pill and Foundayo
- Is Foundayo on the NHS?
- Wegovy pill vs Foundayo
- Weight loss pill prices UK
Compare today's prices: Weight loss pills · Wegovy · Mounjaro
Frequently asked questions
What are the most common side effects of Foundayo?
Nausea, constipation, diarrhoea, vomiting, indigestion and stomach pain. Each affects more than 1 in 10 people, mostly at the start of treatment and after dose increases.
How long do Foundayo side effects last?
For most people they're worst in the first one to two weeks on each new dose and then ease. If they don't settle, your prescriber can keep you on your current dose for longer.
Does Foundayo cause hair loss?
Hair loss is listed as a common side effect, affecting up to 1 in 10 people. It's often linked to fast weight loss and usually settles.
Can I take Foundayo at night to reduce nausea?
Yes. Foundayo can be taken at any time of day, with or without food. Some people find an evening dose easier.
Does Foundayo affect the contraceptive pill?
It may. The UK leaflet advises using condoms or a non-oral method for 30 days after starting and for 30 days after each dose increase.
What should I do if I miss a dose?
Take it as soon as you remember, but never take more than one tablet in a day.
This article is for information only and is not medical advice. CompareTheJab is an independent comparison website, not a pharmacy or medical service. Always speak to a qualified prescriber before starting, stopping or switching any medicine.
Prices were checked on each provider's own website on 3 November 2026 and change often. Always confirm the price at checkout. Some links on this page are affiliate links, which never affect how we rank providers. Read our affiliate policy and how we compare providers.
