Foundayo side effects: what the trials actually showed

If you search "is Foundayo safe", most of what comes back is either marketing gloss or alarm. Neither is much use. What is useful are the numbers from the published phase 3 trials — how often each side effect actually happened, how that compared with placebo, and how many people stopped taking the tablet because of it. This page sets those numbers out, plus the serious risks on the US label and who the medicine is not for.

Where these numbers come from

Foundayo is Eli Lilly's once-daily oral GLP-1 receptor agonist, approved by the US FDA on 1 April 2026 and licensed by the UK MHRA on 10 August 2026. The safety data below comes from ATTAIN-1, a 72-week randomised, double-blind, placebo-controlled trial in 3,127 adults with obesity (or overweight with weight-related conditions, excluding type 2 diabetes), published in the New England Journal of Medicine. A companion trial, ATTAIN-2, tested orforglipron in more than 1,600 adults who also had type 2 diabetes; the dose-by-dose side-effect frequencies on this page, however, are drawn specifically from ATTAIN-1.

One caution before the table: the trials used capsules at doses of 6 mg, 12 mg and 36 mg, while the marketed tablet comes in strengths from 0.8 mg up to 17.2 mg. The milligram numbers differ between the two forms, but the UK SmPC states which tablet strength has an equal effect to each capsule dose: 6 mg matches the 5.5 mg tablet, 12 mg the 9 mg tablet and 36 mg the 17.2 mg tablet. The two forms are not interchangeable milligram for milligram. Our Foundayo dosage guide explains the marketed strengths and the step-up schedule in detail.

The common side effects, with trial frequencies

The dominant side effects were gastrointestinal — the same family of effects seen with every GLP-1 medicine. In ATTAIN-1 they were mostly mild to moderate, and more frequent at higher doses:

Two things are worth noticing in that table. First, the placebo column is not zero — around one in ten people reported nausea, constipation or diarrhoea while taking a dummy tablet, so the drug's true added effect is the gap between the columns, not the raw percentage. Second, vomiting shows the clearest dose relationship, roughly doubling from the lowest to the highest research dose. The US prescribing information pools ATTAIN-1 with ATTAIN-2, so its figures differ slightly: nausea, for example, affected 26%, 34% and 35% of people at the 5.5 mg, 9 mg and 17.2 mg tablet-equivalent doses, against 10% on placebo.

Beyond the big four, the FDA label's table of pooled trial results also lists indigestion, abdominal pain, headache, abdominal swelling or distension, fatigue, belching, heartburn (reflux), gas and hair loss, each more common than on placebo. Hair loss tends to surprise people, but it appears on the label alongside the digestive effects.

How many people stopped taking it

Percentages of people experiencing nausea only tell half the story; what matters just as much is whether the side effects were bad enough to make people quit. In ATTAIN-1, discontinuation because of adverse events was:

• 5.3% at the 6 mg research dose
• 7.9% at 12 mg
• 10.3% at 36 mg
• 2.7% on placebo

Read plainly: even at the highest dose, fewer than one in ten participants stopped specifically because of side effects. That is consistent with the "mostly mild to moderate" description in the published paper, but it is not trivial either — one person in ten stopping over side effects is a real cost, and it is the main reason the dosing is designed the way it is.

Why the dose is increased so slowly

The marketed schedule starts at 0.8 mg once daily and steps up through six strengths to a maximum of 17.2 mg, with at least 30 days at each step. The label's stated rationale is exactly this side-effect profile: gradual escalation reduces the risk of gastrointestinal reactions, and the maintenance dose is chosen on response and tolerability rather than defaulting to the maximum. If nausea at higher doses is the problem, staying longer at a lower step — or settling at a lower maintenance dose — is the standard answer. The full schedule is on our dosage page.

Serious but less common risks

The FDA label carries warnings that apply across the GLP-1 class, plus the standard contraindications. The UK SmPC's warnings cover much the same ground (it adds low blood pressure) but include no thyroid warning. Most of these events are uncommon, but they are the reason this is a prescription medicine and not a supplement:

• Thyroid C-cell tumours (US label only). The US boxed warning notes that thyroid C-cell tumours have been seen in rats and mice given GLP-1 medicines that are active in those animals. Orforglipron is not active in rats or mice and did not cause tumours in rodent studies, and whether the finding matters for humans has not been determined; the label still tells prescribers to counsel patients about a potential risk of medullary thyroid carcinoma (MTC). The UK SmPC carries no thyroid warning.
• Acute pancreatitis. Sudden, severe inflammation of the pancreas.
• Severe gastrointestinal reactions. Beyond ordinary nausea — reactions severe enough to need medical attention.
• Acute kidney injury. Usually secondary to dehydration from prolonged vomiting or diarrhoea, which is why persistent GI symptoms should not simply be endured.
• Gallbladder disease. A recognised effect across GLP-1 medicines, particularly with rapid weight loss.
• Hypoglycaemia. Mainly a risk when combined with insulin or sulfonylureas in people with diabetes.
• Diabetic retinopathy, allergic reactions and anaesthesia. The label also covers complications of diabetic retinopathy in people with type 2 diabetes, serious allergic reactions, and the risk of food or liquid getting into the lungs during general anaesthesia or deep sedation.

Who it is not for

In the UK, the SmPC's only contraindication is allergy to orforglipron or any of the tablet's other ingredients. The US label also rules the medicine out entirely for two groups:

• People with a personal or family history of medullary thyroid carcinoma (MTC).
• People with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).

The UK patient leaflet also says the medicine should not be used during pregnancy. Beyond the outright contraindications, the warnings above mean a prescriber would weigh the medicine carefully for anyone with a history of pancreatitis, gallbladder disease, kidney problems, or diabetes treated with insulin or a sulfonylurea. This is precisely the sort of judgement a website cannot make for you.

How this compares with injectable GLP-1s

At a high level, the tablet's safety profile looks like the class it belongs to. The gastrointestinal effects — nausea, constipation, diarrhoea, vomiting, easing with slow titration — are the same ones reported for the injectable GLP-1 medicines already used in the UK, and the ATTAIN-2 trial reported cardiometabolic changes consistent with the injectable GLP-1s. This page does not rank Foundayo against any specific injectable for tolerability: the frequencies above come from Foundayo's own trials, not from a direct comparison, so treat "better or worse tolerated" claims with caution. What can be said is that being a small-molecule tablet rather than an injected peptide changes how you take it — any time of day, no food or water restrictions — not the fundamental character of its side effects.

The UK situation, and reporting side effects

Foundayo was licensed by the MHRA on 10 August 2026 and has been available in the UK by private prescription since 24 August 2026. It is not available on the NHS, where NICE's appraisal is provisionally expected to publish on 27 January 2027. The UK Summary of Product Characteristics for each strength is now published on the electronic medicines compendium (emc) — for UK patients, that document and the patient information leaflet in the box, rather than the US label summarised above, are the authority on warnings and contraindications. Foundayo is a prescription-only medicine, and anything sold as "Foundayo" without a prescription from a qualified prescriber is being sold outside the law and is potentially dangerous. Our UK availability page tracks the regulatory position.

If you experience a suspected side effect from any medicine — licensed or not — report it through the MHRA Yellow Card scheme. Yellow Card reports are how the UK regulator spots safety signals that trials are too small or too short to catch, and reports about products bought from illegal sellers are especially valuable.

When to seek help

General guidance for anyone taking a GLP-1 medicine, drawn from the risks on the label:

• Severe, persistent abdominal pain, with or without vomiting — possible pancreatitis or gallbladder disease. Seek urgent medical advice.
• Vomiting or diarrhoea you cannot keep under control — dehydration can injure the kidneys. Contact a clinician rather than waiting it out.
• A lump or swelling in the neck, hoarseness, trouble swallowing or shortness of breath — symptoms of thyroid tumours that the US label's warning tells prescribers to explain to patients.
• Signs of low blood sugar (shakiness, sweating, confusion) if you also use insulin or a sulfonylurea.
• Signs of a serious allergic reaction (breathing problems, swelling of the face or throat, wheezing, feeling dizzy or weak) — get immediate medical help.

None of this replaces the advice of your GP or pharmacist, who can see your history and the label in force at the time.

Common questions

Further reading

• Foundayo side effects: what the UK leaflet and trials report
• Orforglipron trial results: what ATTAIN and ACHIEVE found
• Foundayo dosage schedule: the dose steps month by month