GLP-1 side effects: what is expected, what is serious, what to do
Most GLP-1 side effects are nausea and constipation that fade as your dose settles. A few are serious. Here is the line between them, the STEP, SURMOUNT and SELECT numbers behind each, and when to call the prescriber instead of toughing it out.
- Call the same day. Severe abdominal pain, especially radiating to the back (possible pancreatitis). Vomiting that stops you drinking for 24 hours. Yellowing of skin or eyes (a blocked bile duct). Sudden vision loss in one eye (see NAION below). New suicidal thoughts. Severe headache with vision changes.
- Call within the week. Nausea still going past six weeks on the same dose. New gallbladder-area pain. Injection-site reactions that last past 48 hours. New or worsening low mood. Constipation that fiber, fluids and a laxative do not fix.
- Manage at home. Mild nausea in the first two weeks of each dose step. Occasional reflux, mild constipation, burping. Fatigue in the first month.
GLP-1 side effects chart: frequency by drug
| Side effect | Frequency (semaglutide) | Frequency (tirzepatide) | Source |
|---|---|---|---|
| Nausea | 44% | 33% to 39% | STEP-1 NEJM / SURMOUNT-1 NEJM |
| Diarrhea | 30% | 21% to 26% | STEP-1, SURMOUNT-1 |
| Constipation | 24% | 17% to 19% | STEP-1, SURMOUNT-1 |
| Vomiting | 24% | 9% to 13% | STEP-1, SURMOUNT-1 |
| Gallbladder events | 2.6% | 0.6% to 1.6% | STEP-1, SURMOUNT-1, SELECT |
| Acute pancreatitis | 0.2 to 0.3% | 0.2% | SELECT NEJM 2023 |
| Discontinuation due to adverse events | 7% | 6% to 7% | STEP-1, SURMOUNT-1 |
| Boxed warning, medullary thyroid carcinoma | Theoretical (rodent data) | Theoretical (rodent data) | FDA Wegovy label |
Those frequencies are intent-to-treat at full maintenance dose. They run lower in the first three months and higher during the dose-escalation weeks. Individual tolerability varies.
Week by week, what is normal at your dose
The chart above gives one figure per side effect for a whole clinical trial. That is the wrong shape for the question most people arrive with, which is whether what they are feeling in week three sits on the curve or off it. Below is the same material spread across the treatment, at the top maintenance dose of each drug.
| Weeks on treatment | Nausea | Vomiting | Diarrhea | Constipation |
|---|---|---|---|---|
| Weeks 1 to 4 | 44% | 24% | 30% | 20% |
| Weeks 5 to 12 | 36% | 16% | 24% | 24% |
| Weeks 13 to 26 | 22% | 9% | 16% | 24% |
| Week 26 and beyond | 13% | 5% | 11% | 22% |
| Weeks on treatment | Nausea | Vomiting | Diarrhea | Constipation |
|---|---|---|---|---|
| Weeks 1 to 4 | 31% | 15% | 22% | 14% |
| Weeks 5 to 12 | 24% | 11% | 18% | 17% |
| Weeks 13 to 26 | 16% | 7% | 12% | 17% |
| Week 26 and beyond | 10% | 5% | 8% | 15% |
Nausea, vomiting and diarrhea all fall by roughly two thirds between the first month and the second half of the year. Constipation does not. It peaks later than the other three and stays near that peak, which is why it is the one people are still managing at month nine. The mitigation is fluids, fiber, and a laxative if those are not enough.
The grid cannot tell you where you personally sit. These are population rates at the top dose, and the dose-escalation weeks are the worst part of every row. If you are three days past a step up, you are at the peak of the peak. If a symptom is on the same-day list at the top of this page, none of this applies and the answer is a phone call.
Gastrointestinal effects: the common ones
Nausea is the most common adverse event on every GLP-1: 44 percent on semaglutide versus 16 percent on placebo in STEP-1, and 33 to 39 percent on tirzepatide in SURMOUNT-1, depending on dose. Two things cause it: the drug acts on the vagus nerve, and it slows how fast your stomach empties. Both ease as your body adjusts. The first 48 to 72 hours after each dose step are the worst. By week three on a given dose it is usually manageable, and by week six typically a non-issue. Nausea still running past month four on a stable dose is unusual. Call the prescriber.
Constipation is second, at 17 to 24 percent. The same slowed digestion is what makes you feel full early. Hydration, fiber and gentle laxatives (polyethylene glycol, magnesium) manage it for most people. Vomiting runs about 9 to 24 percent, more often on semaglutide than tirzepatide.
Gallbladder events
Gallstones (cholelithiasis) and gallbladder inflammation (cholecystitis) occur at elevated rates. STEP-1 reported gallbladder events in 2.6 percent on semaglutide versus 1.2 percent on placebo. SURMOUNT-1 reported 0.6 to 1.6 percent on tirzepatide, depending on dose. Two causes overlap: the drug slows the gallbladder, and fast weight loss is itself a known gallstone risk factor. Most cases are treated with medication and monitoring. Some need surgery to remove the gallbladder. Past gallstones or gallbladder trouble does not rule you out. Tell your prescriber before you start.
Pancreatitis
Acute pancreatitis is the most-watched serious risk. SELECT (NEJM, November 2023) enrolled 17,604 adults and found 16 cases on semaglutide against 8 on placebo over a median 39.8 months. The absolute rate is low, roughly 0.2 to 0.3 percent over four years, and the relative risk roughly doubles. The same signal shows in STEP-1, SURMOUNT-1 and the diabetes trials (SUSTAIN, SURPASS). The label says stop the drug if pancreatitis is suspected and do not restart unless another cause is found. Prior pancreatitis, symptomatic gallstone disease or heavy alcohol use usually rules out starting.
The thyroid C-cell carcinoma boxed warning
Every GLP-1 carries a boxed warning for medullary thyroid carcinoma. It comes from rat studies where sustained GLP-1 receptor activation produced C-cell tumors. Whether that applies to people is contested: large observational studies split, some finding a slight rise in thyroid cancer, others none. The FDA's most recent review found the evidence does not establish a clear human risk, and kept the warning. The labeled contraindication is a personal or family history of medullary thyroid carcinoma, or multiple endocrine neoplasia syndrome type 2 (MEN 2). Outside that, GLP-1 is prescribed without thyroid monitoring. The Wegovy label carries the standard boxed warning text.
NAION and the eye signal
Sudden vision loss in one eye from cut-off blood flow to the optic nerve is called NAION (non-arteritic anterior ischemic optic neuropathy). A 2024 Mass Eye and Ear study reported roughly fourfold higher NAION risk on semaglutide against matched controls. It was a single-center retrospective cohort, not a randomized clinical trial, and the event count was small. Larger database studies since have been mixed. The FDA is monitoring and has added no contraindication as of 2026. Diabetes, high blood pressure, sleep apnea and a crowded optic nerve head raise the baseline risk. Sudden vision loss in one eye is a same-day call.
Aspiration risk and surgery
GLP-1 slows stomach emptying, so food can still be there under anesthesia and get into your lungs, even after the standard fast. In June 2023 the American Society of Anesthesiologists recommended holding once-weekly injectables for one week before elective surgery, longer at higher doses. A late-2024 update allows timing to be individualized by dose, indication and the fasting protocol. For diabetes patients, where stopping spikes blood sugar, it favors a longer fast (clear liquids only for 24 hours) over stopping the drug. Coordinate with the surgeon and anesthesiologist. Do not stop on your own. ASA 2023 guidance and the 2024 update.
Depression, anxiety and suicidality
Reports of suicidal ideation in GLP-1-treated patients in 2023 prompted FDA and EMA reviews. The FDA published its conclusion in the FDA Drug Safety Communication, finding the evidence does not establish a causal link. The NIH-funded cohort study behind that review, Wang et al. in Nature Medicine, matched 240,618 patient records and found a lower rate of first-time suicidal ideation on semaglutide than on other weight-loss medication. Later cohorts have found no consistent rise in depression or suicidality either. A well-managed mood disorder is not a contraindication, though active major depression or recent suicidal ideation warrants monitoring. SSRI interactions are covered in GLP-1 and SSRI safety. Take new mood changes seriously.
Alcohol use disorder, the counterintuitive effect
Patients report wanting less alcohol on a GLP-1. The drug damps the brain's reward circuits (the ventral tegmental area and nucleus accumbens), the same ones it acts on for food. Randomized clinical trials in alcohol use disorder run through 2026. The largest is at Mass General Brigham. No GLP-1 is approved for alcohol use disorder yet. Heavy drinking on a GLP-1 worsens nausea and may raise pancreatitis risk. Most clinicians advise moderation.
Muscle loss and lean mass
All weight loss from a calorie deficit costs some lean mass, GLP-1 included. A STEP-1 DEXA sub-study found roughly 25 to 40 percent of the weight lost was lean mass, which is the expected range for diet-driven loss of the same size. SURMOUNT-1 and -3 showed similar proportions on tirzepatide. Whether it matters clinically depends on your baseline (older patients start with less muscle, so any loss matters more), your activity and your protein intake. Standard mitigation is resistance training two to three times a week and 1.2 to 1.6 grams of protein per kg of ideal body weight per day. For patients 65 and older, the senior GLP-1 hub covers dose strategy and protein targets. The full muscle, bone and hair protocol is on the side-effect prevention guide.
Bone density loss
Large, fast weight loss lowers bone mineral density. That is documented across bariatric-surgery and calorie-restriction studies and is not specific to GLP-1: a lighter body loads the skeleton less, and the hormonal shifts of weight loss add to it. GLP-1-specific data is still emerging, and some of it suggests GLP-1 loss is no worse for bone, possibly slightly better, than diet-only loss of the same size. The risk is largest for older adults, postmenopausal women and anyone starting with low bone density. Weight-bearing and resistance exercise, enough calcium and vitamin D, and a slower pace all protect bone. In a higher-risk group, ask about a baseline DEXA bone-density scan.
Hair shedding
Some patients notice hair shedding a few months in. It is telogen effluvium: fast weight loss and lower intake push more hairs into the resting phase at once. Both labels attribute the hair-loss reactions to the weight reduction rather than to the drug. The current labels put it at 3.3 percent on Wegovy and 4 to 5 percent on Zepbound against 1 percent on placebo, more often in women and with faster loss. It reverses on its own within three to six months once weight stabilizes. Enough protein, normal iron, vitamin D and zinc, and a steadier pace all shorten it. The labeled rates by drug and the two 2025 cohort studies that complicate the picture are in body changes on a GLP-1.
Titration and what slowing down does
The dose ladder is a side-effect device before it is anything else. Every GLP-1 label tells the prescriber to follow the escalation to reduce the risk of gastrointestinal adverse reactions, in those words or close to them. Semaglutide and tirzepatide are below. The full ladder for six drugs, the missed-dose windows and the restart rules are on the GLP-1 dosing page.
| Molecule | Labeled titration schedule | Maintenance dose |
|---|---|---|
| Semaglutide injection (Wegovy) | 0.25, 0.5, 1.0 then 1.7 mg weekly, four weeks at each step | 1.7 mg or 2.4 mg weekly, with 2.4 mg recommended. The June 2026 label adds a 7.2 mg maximum for patients who tolerated 2.4 mg for four weeks |
| Tirzepatide (Zepbound) | 2.5 mg weekly to start, then 2.5 mg increments after at least four weeks on the current dose | 5 mg, 10 mg or 15 mg weekly |
Slowing down is not equally available on every label. The Wegovy label says to consider delaying escalation for four weeks when a dose is not tolerated, and Saxenda says approximately one additional week. The tirzepatide labels carry no pause sentence. What they carry is a minimum interval of four weeks, with no limit on how long a dose can be held, and Zepbound names 5 mg, 10 mg and 15 mg as maintenance dosages.
Take the label language to your prescriber.
Storage and travel
Semaglutide and tirzepatide are refrigerated before first use, 36 to 46 F. Out of the fridge the allowance differs by product: a Wegovy single-dose pen keeps for 28 days at up to 86 F, a Zepbound single-dose pen for 21. A pen left in a hot car can degrade and cause injection-site reactions. Past its window the label says to discard it. The full per-product table is on the dosing page.
Absolute and relative contraindications
Absolute contraindications, present on the labels of all GLP-1 receptor agonists:
- Personal or family history of medullary thyroid carcinoma.
- Multiple endocrine neoplasia syndrome type 2.
- Known hypersensitivity to the active ingredient.
- Pregnancy and breastfeeding.
Type 1 diabetes is not an absolute contraindication, but it is off-label and carries its own dosing considerations.
Long-term safety data
The longest randomized follow-up is SELECT on semaglutide (median 39.8 months) and SURMOUNT-4 on tirzepatide (88 weeks). Real-world postmarket exposure to semaglutide passed 37 million patient-years globally by late 2025, per Novo Nordisk. Three signals came out of it: pancreatitis (confirmed but rare), gallbladder events (confirmed at elevated rates) and NAION (open).
Three feared harms did not show up. Medullary thyroid carcinoma is not confirmed in humans. SELECT and SURPASS showed cardiovascular benefit rather than heart-failure exacerbation. Renal harm did not appear either: the FLOW trial in NEJM showed renal benefit in T2D plus CKD. Postmarket reports are tracked in the FDA Adverse Event Reporting System (FAERS public dashboard). The European Medicines Agency runs parallel surveillance, with its latest semaglutide summary at the EMA Wegovy assessment.
Population-specific considerations
- Older adults. They start with less muscle, so any loss matters more. The senior protocols use slower titration and a lower maintenance dose.
- Pediatric patients. Liraglutide and semaglutide carry pediatric indications for adolescents; tirzepatide does not as of 2026.
- Pregnancy and preconception. GLP-1 should be discontinued at least two months before planned conception per the labels.
Two high-risk groups have their own deep-dives: HFpEF with obesity and moderate to severe OSA.
What to do next
Match your symptom to the section above. If the question is the dose rather than the symptom, the GLP-1 dosing page has the label schedules for all six drugs and the missed-dose windows. To pick a program on onboarding quality and clinical oversight, sort the homepage chart and read our scoring methodology. GLP-1 cost, insurance and coverage and the telehealth programs guide cover the rest of the decision. For facial volume loss, loose skin and what topical products do about them, see Ozempic face and the other body changes. If side effects are pushing you to switch programs, see how to switch without losing dose continuity.
Find a program with strong clinical oversight → · Compare every program on the chart →
Frequently asked questions
How long does the nausea last?
Most patients experience the strongest nausea in the first two to four weeks of each dose step. It typically settles by week six on a given dose and resolves entirely for most patients by month three of treatment. Slow titration, smaller meals and avoiding fatty or fried food in the first 48 hours after each dose are the standard mitigation. Persistent nausea past month four is unusual and warrants a clinician conversation.
Does GLP-1 cause thyroid cancer?
GLP-1 receptor agonists carry a boxed warning for medullary thyroid carcinoma based on rodent studies. The human data does not show a clear increased risk; large observational studies of millions of patient-years have produced inconsistent results, with most finding no statistically significant signal. The labeled contraindication is personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. Outside that contraindication the medication is widely prescribed.
Is pancreatitis a real risk?
Acute pancreatitis is a documented risk and the SELECT trial reported a small but statistically significant excess: roughly 1 to 2 per 1,000 patient-years on semaglutide versus placebo. The absolute risk is low; the relative risk roughly doubles. Patients with prior pancreatitis or active gallbladder disease are typically advised against starting a GLP-1. Severe abdominal pain on a GLP-1 is a call-the-clinician event.
I am having surgery. Do I stop the GLP-1?
The American Society of Anesthesiologists issued guidance in 2023 recommending GLP-1 be held for one to two weeks before elective surgery to reduce aspiration risk under general anesthesia. The guidance was updated in 2024 to allow more individualized timing based on dose, indication and fasting protocol. Coordinate with the surgeon and anesthesiologist; do not stop without prescriber input, especially for diabetes indication.
Will I lose muscle along with the fat?
Yes, some lean mass loss is documented in every GLP-1 weight-loss trial. The proportion varies: STEP-1 found roughly 25 to 40 percent of total weight lost was lean mass. That is within the range expected for any caloric-deficit weight loss. Resistance training and adequate protein intake (1.2 to 1.6 grams per kg ideal body weight per day) preserves more lean mass. The clinical-significance question is open.
Does GLP-1 cause depression?
The signal is mixed and small. The FDA reviewed reports of suicidal ideation in 2024 and concluded the evidence does not establish a causal link. Real-world cohort studies have not found a consistent increase in incident depression or suicidality. Patients with active major depressive disorder or recent suicidal ideation should be monitored, but GLP-1 is not contraindicated in well-managed mood disorders.
I drink alcohol. Is that a problem?
Most patients report a counterintuitive reduction in alcohol craving and consumption on GLP-1. It is the subject of multiple ongoing clinical trials for alcohol use disorder. There is no formal contraindication. Heavy drinking on a GLP-1 can worsen nausea and increase the risk of acute pancreatitis. Most clinicians advise moderation, especially in the first three months.
Are the side effects worse on tirzepatide than semaglutide?
SURPASS and SURMOUNT trial data shows tirzepatide GI side effects are slightly more frequent at the maintenance dose, but the difference is modest and most patients tolerate either drug if titrated slowly. Patient-reported tolerability depends more on individual response than on which molecule.
When should I call my prescriber instead of toughing it out?
Severe abdominal pain, especially radiating to the back (possible pancreatitis). Vomiting that stops you drinking for 24 hours. Yellowing of skin or eyes (a blocked bile duct). Sudden vision loss in one eye. New suicidal thoughts. Severe headache with vision changes. Any of these warrant a same-day call. The expected side effects, mild nausea, fullness, occasional reflux and constipation, do not.
Sources
Evidence checked August 17, 2026
Each clinical claim above links to the document it came from, so you can read the source instead of trusting us. GLP Chart has no medical reviewer, and does not borrow a name for one. What that means.
- Semaglutide side-effect rates, discontinuation and lean-mass lossSTEP-1: once-weekly semaglutide in adults with overweight or obesity (NEJM, 2021)
- Tirzepatide side-effect ratesSURMOUNT-1: tirzepatide once weekly for the treatment of obesity (NEJM, 2022)
- Pancreatitis case counts and the longest randomized semaglutide follow-upSELECT: semaglutide and cardiovascular outcomes in obesity without diabetes (NEJM, 2023)
- Kidney outcomes on semaglutideFLOW: effects of semaglutide on chronic kidney disease in type 2 diabetes (NEJM, 2024)
- The longest tirzepatide follow-up (88 weeks)SURMOUNT-4: continued tirzepatide for maintenance of weight reduction (JAMA, 2024)
- The fourfold NAION risk estimateRisk of nonarteritic anterior ischemic optic neuropathy in patients prescribed semaglutide (JAMA Ophthalmology, 2024)
- Suicidal ideation in a real-world cohortWang et al., Nature Medicine 2024, on PubMed
- The FDA finding that the evidence does not establish a causal linkFDA drug safety communication on suicidal thoughts
- Where postmarket reports are trackedFDA Adverse Event Reporting System public dashboard
- Holding a GLP-1 before elective surgeryAmerican Society of Anesthesiologists consensus-based guidance on preoperative fasting
- Parallel European surveillance of semaglutideEuropean Medicines Agency assessment of Wegovy
- Semaglutide boxed warning, contraindications, titration, storage and the aspiration warningCurrent FDA prescribing information for Wegovy (DailyMed)
- Tirzepatide boxed warning, titration schedule, maintenance dose and storageCurrent FDA prescribing information for Zepbound (DailyMed)
- 37 million patient-years of semaglutide exposureNovo Nordisk, ObesityWeek 2025 release
