Foundayo Side Effects: Common, Serious, and What to Watch For

Foundayo (orforglipron) is a once-daily GLP-1 pill from Eli Lilly that the FDA approved on April 1, 2026 for long-term weight management in adults with obesity, or overweight with at least one weight-related health condition ^{[1] [2]}. Its most common side effects are digestive complaints such as nausea, constipation, diarrhea, vomiting, indigestion, and stomach pain, plus headache, fatigue, and hair loss. The prescribing information lists all of these as reactions reported in 5% or more of Foundayo-treated patients ^[1]. In trials, the adverse-event profile has been consistent with that of other GLP-1 receptor agonists ^[9], and a Nature Medicine paper on orforglipron characterizes its safety as generally similar to injectable GLP-1 receptor agonists ^[10].

The gastrointestinal events in these trials were mostly mild to moderate, and in the 72-week ATTAIN-1 trial, side effects led 5.3 to 10.3% of orforglipron patients to stop treatment, compared with 2.7% of placebo patients ^[9]. The label also lists serious warnings and precautions: a boxed warning about thyroid C-cell tumors, plus pancreatitis, severe stomach reactions, kidney injury from fluid loss, low blood sugar when combined with insulin or an insulin secretagogue, allergic reactions, gallbladder disease, and lung complications during deep sedation or general anesthesia ^[1].

What is Foundayo and how is it taken?

Foundayo is a once-daily small-molecule (non-peptide) oral GLP-1 receptor agonist ^[2]. It was discovered by Chugai Pharmaceutical and licensed by Lilly in 2018 ^[2]. The FDA approved it under NDA 220934, which the agency received on January 20, 2026 ^[5].

The approved use is alongside a reduced-calorie diet and increased physical activity, to reduce excess body weight and keep it off long term in adults with obesity, or adults with overweight who have at least one weight-related comorbid condition ^{[1] [2]}. It is not approved for type 2 diabetes; Lilly has said it plans to submit Foundayo for that use, but that submission had not been approved as of the ACHIEVE-4 announcement on April 16, 2026 ^[6].

The tablet is taken once daily, any time of day, with or without food. Tablets must be swallowed whole — not broken, crushed, or chewed — and no more than one tablet should be taken per day ^{[1] [2]}. The current label is dated Revised 7/2026 ^[1].

What are the most common Foundayo side effects?

The prescribing information lists the most common adverse reactions as those reported in 5% or more of Foundayo-treated patients: nausea, constipation, diarrhea, vomiting, dyspepsia (indigestion), abdominal pain, headache, abdominal distension (a swollen or bloated belly), fatigue, eructation (belching), gastroesophageal reflux disease (heartburn or acid reflux), flatulence (gas), and hair loss ^[1]. Lilly’s consumer materials use the same list in plainer words: nausea, constipation, diarrhea, vomiting, indigestion, stomach pain, headache, swollen belly, feeling tired, belching, heartburn, gas, and hair loss ^[2].

The label excerpt does not break these reactions down by dose or give exact percentages for each one, so no per-dose frequency numbers have been published in the material reviewed here. What the trials do say is consistent: in ATTAIN-1, published in the New England Journal of Medicine, the most common adverse events were gastrointestinal, and those gastrointestinal events were mostly mild to moderate ^[9]. The phase 3b ATTAIN-MAINTAIN trial reported the same pattern — gastrointestinal events, mostly mild to moderate ^[10]. In ACHIEVE-4, the most common adverse events for patients taking Foundayo were nausea, vomiting, diarrhea, decreased appetite, and constipation ^[6].

Does the dose affect side effects?

The label builds in slow dose increases, and each step up is allowed only after at least 30 days on the current dose, based on treatment response and tolerability ^[1].

Step	Dose	When it can be started
Starting dose	0.8 mg once daily	Day 1
Second dose	2.5 mg once daily	After at least 30 days on 0.8 mg
Third dose	5.5 mg once daily	After at least 30 days on 2.5 mg
Higher doses	9 mg, 14.5 mg, or 17.2 mg once daily	After at least 30 days on the current dose
Maximum	17.2 mg once daily	—

All figures come from the prescribing information ^[1]. Tablets come in exactly these strengths: 0.8 mg, 2.5 mg, 5.5 mg, 9 mg, 14.5 mg, and 17.2 mg ^[1].

One point of possible confusion: the ATTAIN-1 trial studied capsule doses of 6 mg, 12 mg, and 36 mg ^[9], and ACHIEVE-4 started participants on a 1 mg capsule, which Lilly states corresponds to the 0.8 mg tablet, escalating every four weeks up to a 36 mg capsule, which corresponds to the 17.2 mg tablet ^[6]. Lilly gives no tablet equivalents for the 6 mg and 12 mg capsule doses used in ATTAIN-1.

Any decision to change, pause, or slow a dose belongs with a prescriber. The label also contains a section titled “Recommendations Regarding Missed Dose,” but its text is not included in the excerpt reviewed here, so dose-gap questions should go to the prescriber or the full prescribing information at uspl.lilly.com/foundayo ^[1].

How many people stop Foundayo because of side effects?

In ATTAIN-1 — a 72-week trial that randomized 3,127 adults with obesity but without diabetes to orforglipron 6 mg, 12 mg, or 36 mg capsules or placebo — adverse events caused treatment discontinuation in 5.3 to 10.3% of the orforglipron groups, compared with 2.7% of the placebo group ^[9].

In ACHIEVE-4, which studied adults with type 2 diabetes and obesity or overweight at increased cardiovascular risk, 10.6% of Foundayo patients discontinued treatment due to adverse events during the 52-week minimum treatment period ^[6].

What is the boxed warning about thyroid tumors?

Foundayo’s label carries a boxed warning about the risk of thyroid C-cell tumors ^[1]. The reasoning behind it is specific to orforglipron. In GLP-1 products that are pharmacologically active in rats and mice, rodent thyroid C-cell tumors (both adenomas and carcinomas) have been seen at clinically relevant exposures, and those tumors are considered GLP-1 receptor-dependent effects in rodents ^[1].

Orforglipron is different in one key way: it is not pharmacologically active in rats or mice, and it did not produce tumors in rodents ^[1]. However, it is pharmacologically active at the human GLP-1 receptor, and the human relevance of the rodent tumor findings has not been determined ^[1]. Because of that open question, the warning stays on the label.

The label also states that routine monitoring of serum calcitonin or thyroid ultrasound is of uncertain value for early detection of medullary thyroid carcinoma (MTC) in patients treated with Foundayo ^[1]. Symptoms to report to a health care provider include a lump or swelling in the neck, trouble swallowing (dysphagia), trouble breathing (dyspnea), and persistent hoarseness ^{[1] [2]}.

What other serious warnings does Foundayo carry?

Beyond the boxed warning, the prescribing information lists these warnings and precautions ^[1]:

• Acute pancreatitis. It has been observed in patients treated with GLP-1 receptor agonists, including Foundayo, and the drug should be discontinued if pancreatitis is suspected. Lilly’s patient materials describe the warning sign as severe abdominal pain that will not go away, with or without nausea or vomiting, sometimes radiating to the back ^[2].
• Severe gastrointestinal reactions. Use has been associated with GI reactions that are sometimes severe, and Foundayo is not recommended in patients with severe gastroparesis ^[1].
• Acute kidney injury from volume depletion. Diarrhea, nausea, and vomiting can cause fluid loss that leads to kidney problems; the label says to monitor renal function in patients reporting reactions that could lead to volume depletion ^{[1] [2]}.
• Hypoglycemia. The risk rises when Foundayo is used with insulin or an insulin secretagogue, and reducing the dosage of those other medicines may be necessary ^[1]. Lilly’s consumer materials name sulfonylureas as an example of such a medicine ^[2].
• Hypersensitivity reactions. Serious reactions including anaphylaxis and angioedema have been reported with GLP-1 receptor agonists; if one is suspected, the patient should promptly seek medical attention and discontinue Foundayo ^[1].
• Diabetic retinopathy complications. Patients with a history of diabetic retinopathy should be monitored for progression ^[1].
• Acute gallbladder disease. This has been reported in clinical trials; if cholecystitis is suspected, gallbladder studies and clinical follow-up are indicated. Symptoms may include upper abdominal pain, fever, jaundice, and clay-colored stools ^{[1] [2]}.
• Pulmonary aspiration during anesthesia or deep sedation. This has been reported in patients receiving GLP-1 receptor agonists undergoing elective surgeries or procedures; patients should tell their providers about any planned surgeries or procedures ^[1].

Suspected adverse reactions can be reported to Eli Lilly at 1-800-LillyRx (1-800-545-5979) or to the FDA at 1-800-FDA-1088 ^[1].

Which drugs interact with Foundayo?

Foundayo is processed in the body through pathways that several common medicines also use, so the label sets hard limits on some combinations ^[1].

Interacting drug or drug type	What the label says
Strong CYP3A4 inhibitors	Maximum Foundayo dosage is 9 mg once daily when used together
Strong CYP3A4 inhibitors that also inhibit OATP1B	Avoid
Strong CYP3A4 inducers	Avoid
Moderate CYP3A4 inducers	Monitor Foundayo effectiveness and escalate dosage as needed
Simvastatin	Do not exceed simvastatin 20 mg once daily
Oral contraceptives	Switch to a non-oral method or add a barrier method for 30 days after starting Foundayo and for 30 days after each dose increase

All of these come from the prescribing information ^[1]. The oral contraceptive guidance matters because Foundayo delays gastric emptying, which has the potential to affect how other oral medications are absorbed ^[1].

The label excerpt reviewed here does not single out antibiotics as a category. Because the interaction rules turn on whether a specific drug inhibits or induces CYP3A4 or affects OATP1B, the practical step is to give the prescriber and pharmacist a complete list of everything being taken, including statins, and let them check it against the full prescribing information ^[1].

Who should not take Foundayo?

The label lists clear contraindications. Foundayo should not be used by anyone with a personal or family history of medullary thyroid carcinoma, or anyone with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2) ^{[1] [2]}. It is also contraindicated in anyone with a known serious hypersensitivity to orforglipron or any of the tablet’s inactive ingredients ^{[1] [2]}.

Beyond the contraindications, the label says concomitant use with another GLP-1 receptor agonist is not recommended ^{[1] [2]}. In pregnancy, Foundayo may cause fetal harm and should be discontinued when pregnancy is recognized ^[1]. It is not recommended for patients with severe hepatic impairment ^[1]. And it is not known whether Foundayo is safe and effective in children ^{[1] [2]}.

On the pediatric question, the FDA approval letter spells out what comes next. Lilly must complete an ongoing study in adolescents ages 12 to under 18 (ADVANCE-ATTAIN-ADOLESCENTS), with study completion in March 2028, and a study in children ages 6 to under 12, with study completion in March 2031. The pediatric requirement was waived for ages birth to under 6 ^[5].

What safety studies did the FDA require after approval?

The FDA determined that spontaneous adverse-event reporting would not be enough to assess the signal of the serious risk of medullary thyroid cancer, or to identify unexpected serious risks of long-term pediatric use and exposure during pregnancy ^[5]. So it imposed postmarketing requirements ^[5]:

• 4977-3: a disease-based registry for pediatric obesity, with a cohort study of at least 10 years tracking growth, development, and adverse events; final report due April 2039.
• 4977-4: a prospective global pregnancy exposure registry comparing maternal, fetal, and infant outcomes between women exposed to orforglipron and unexposed comparators.
• 4977-5: an additional pregnancy study using a different design, such as a claims or electronic medical record cohort.

Reuters reported on April 14, 2026 that the FDA required post-marketing studies for the newly approved obesity pill ^[11]. The approval letter also notes the application was not referred to an FDA advisory committee because no controversial issues were identified ^[5].

What do longer-term trials show about Foundayo’s safety?

ACHIEVE-4, announced April 16, 2026, is the longest Phase 3 study of Foundayo to date. It was an open-label trial that randomized 2,749 participants across 15 countries — adults with type 2 diabetes and obesity or overweight at increased cardiovascular risk — to escalating doses of Foundayo or insulin glargine, and it ran through 104 weeks of therapy with a 52-week minimum treatment period ^[6].

On cardiovascular safety, Foundayo met its primary objective of non-inferiority: the hazard ratio for MACE-4 (cardiovascular death, heart attack, stroke, or hospitalization for unstable sudden chest pain) was 0.84 (95% CI 0.59 to 1.20), meeting the prespecified criterion of an upper confidence limit below 1.8. The MACE-3 hazard ratio was 0.77 (95% CI 0.52 to 1.13). In a pre-planned analysis not controlled for multiplicity, all-cause death showed a hazard ratio of 0.43 (95% CI 0.25 to 0.75) ^[6].

The trial included a thorough analysis of potential drug-induced liver injury, and those analyses confirmed there was no hepatic safety signal, consistent with all prior studies in the ACHIEVE and ATTAIN programs ^[6]. Lilly has stated that across seven Phase 3 studies enrolling more than 11,000 patients, Foundayo has shown a consistent safety and efficacy profile ^[6]. The ATTAIN program alone enrolled more than 4,500 people across ATTAIN-1 and ATTAIN-2, each running 72 weeks ^[2].

What do patient reviews say about side effects?

Early user reviews exist, but they are anecdotal, self-reported, and not verified, so they cannot show how often any side effect really happens. Drugs.com shows an average rating of 7.0 out of 10 from 40 reviews, with 50% of reviewers reporting a positive experience and 13% a negative one ^[7]. The individual reviews visible on the retrieved page are dated May through September 2026 ^[7]. The site’s “commonly mentioned side effects” tags are nausea, constipation, and fatigue ^[7].

Individual reviews describe nausea in the first few days that then resolved, constipation, fatigue or exhaustion, a headache after a dose step-up from 0.8 mg to 2.5 mg, and one emergency room visit with a colitis diagnosis after moving from 0.8 mg to 2.5 mg ^[7]. With only 40 reviews, none of this is evidence of frequency or causation — the label’s 5%-or-higher list and the trial data above are the reliable sources ^{[1] [9]}.

When should you call a doctor about Foundayo side effects?

Lilly’s patient materials give specific stop-and-call guidance ^[2]:

• Severe abdominal pain that will not go away, with or without nausea or vomiting, sometimes felt from the abdomen through to the back — stop taking Foundayo and call the provider right away, because this can signal pancreatitis.
• Nausea, vomiting, or diarrhea that does not go away — tell the provider right away, and drink fluids to reduce the chance of dehydration and kidney problems.
• Signs of a serious allergic reaction — swelling of the face, lips, tongue, or throat; trouble breathing or swallowing; severe rash or itching; fainting or dizziness; or a very rapid heartbeat. Stop the drug and get medical help right away.
• Signs of low blood sugar — dizziness or light-headedness, sweating, confusion or drowsiness, headache, blurred vision, slurred speech, shakiness, fast heartbeat, anxiety, irritability, mood changes, hunger, weakness, or feeling jittery.
• Signs of gallbladder problems — pain in the upper stomach, fever, yellowing of the skin or eyes (jaundice), or clay-colored stools.
• A lump or swelling in the neck, hoarseness, trouble swallowing, or shortness of breath — possible thyroid tumor symptoms.

Patients should also tell every health care provider about planned surgeries or procedures before they happen, because of the pulmonary aspiration warning ^[1]. The Medication Guide and full Prescribing Information are available at uspl.lilly.com/foundayo ^{[1] [2]}.

Sources

1. Eli Lilly and Company. FOUNDAYO (orforglipron) tablets, for oral use. Prescribing information. Initial U.S. Approval: 2026.
2. Eli Lilly and Company. FDA approves Lilly’s Foundayo (orforglipron), the only GLP-1 pill for weight loss that can be taken any time of day without food or water restrictions. April 1, 2026.
3. The Lancet. Efficacy and safety of once-daily oral orforglipron compared with oral semaglutide in adults with type 2 diabetes (ACHIEVE-3): a multinational, multicentre, non-inferiority, open-label, randomised, phase 3 trial.
4. Reddit. Reddit discussion thread on Wegovy pill weight loss results.
5. US Food and Drug Administration. Foundayo (orforglipron) tablets. NDA approval letter.
6. Eli Lilly and Company. ACHIEVE-4, the longest Phase 3 study of Lilly’s Foundayo (orforglipron) to date, reaffirmed its cardiovascular and overall safety profile. April 16, 2026.
7. Drugs.com. Foundayo Reviews & Ratings.
8. Reddit. Reddit discussion thread on one month of Foundayo use.
9. PubMed. Orforglipron, an Oral Small-Molecule GLP-1 Receptor Agonist for Obesity Treatment.
10. Nature Medicine. Orforglipron for maintenance of body weight reduction: the double-blind, randomized phase 3b ATTAIN-MAINTAIN trial.
11. Reuters. FDA seeks additional safety data from Lilly on newly approved weight-loss pill. April 14, 2026.