Mounjaro Side Effects: What the Label Reports and How Often
The most common Mounjaro side effects are nausea, diarrhea, decreased appetite, vomiting, constipation, dyspepsia, and abdominal pain; nausea reached 18% at the 15 mg dose in placebo-controlled trials. The label also carries a boxed warning about thyroid C-cell tumors seen in rats.
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The most common side effects the US Mounjaro (tirzepatide) prescribing information reports are nausea, diarrhea, decreased appetite, vomiting, constipation, dyspepsia (indigestion), and abdominal pain, each occurring in 5% or more of treated patients [1]. Most of these are gastrointestinal, and they cluster around dose increases: the label states that the majority of nausea, vomiting, and diarrhea reports happened during dose escalation and decreased over time [1]. The label also carries a boxed warning about thyroid C-cell tumors seen in rats, and it lists pancreatitis, low blood sugar when combined with insulin or sulfonylureas, kidney injury, gallbladder disease, and severe GI reactions among its warnings [1].
Mounjaro is approved in the US to improve glycemic control in adults and pediatric patients 10 years and older with type 2 diabetes, and to reduce the risk of major adverse cardiovascular events in adults with type 2 diabetes at high risk for those events [1]. The label was revised in August 2026 [1]. Zepbound contains the same molecule, tirzepatide, but is approved for weight reduction and for moderate to severe obstructive sleep apnea in adults with obesity [2]. In the UK, research from University College London, cited by the MHRA, estimates that 1.6 million adults in England, Wales and Scotland used GLP-1 medicines such as semaglutide and tirzepatide for weight loss between early 2024 and early 2025 [9].
How common are the most common side effects at each dose?
The label’s Table 1 comes from a pool of two placebo-controlled trials in adults with type 2 diabetes (SURPASS-1 and SURPASS-5), covering 718 patients exposed to Mounjaro with a mean exposure of 36.6 weeks [1]. Rates rose with dose for most reactions [1].
| Adverse reaction | Placebo | 5 mg | 10 mg | 15 mg |
|---|---|---|---|---|
| Nausea | 4% | 12% | 15% | 18% |
| Diarrhea | 9% | 12% | 13% | 17% |
| Decreased appetite | 1% | 5% | 10% | 11% |
| Vomiting | 2% | 5% | 5% | 9% |
| Constipation | 1% | 6% | 6% | 7% |
| Dyspepsia | 3% | 8% | 8% | 5% |
| Abdominal pain | 4% | 6% | 5% | 5% |
All figures are from the label’s Table 1 [1]. Looking at GI reactions as a group, any gastrointestinal adverse reaction occurred in 20.4% of placebo patients, 37.1% at 5 mg, 39.6% at 10 mg, and 43.6% at 15 mg [1]. Less frequent GI reactions that were still more common than placebo included eructation (belching), flatulence, gastroesophageal reflux disease, and abdominal distension [1].
When do side effects start and how long do they last?
The recommended starting dosage is 2.5 mg once weekly, with increases in 2.5 mg steps after at least 4 weeks on the current dose, and the label states this escalation schedule is intended to reduce the risk of GI reactions [1]. The majority of nausea, vomiting, and diarrhea reports occurred during dose escalation and decreased over time [1].
Some patients stopped treatment because of GI reactions: 3.0% at 5 mg, 5.4% at 10 mg, and 6.6% at 15 mg, compared with 0.4% on placebo [1].
What is the boxed warning about thyroid tumors?
Mounjaro’s boxed warning states that in both male and female rats, tirzepatide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures [1]. It is unknown whether Mounjaro causes thyroid C-cell tumors, including medullary thyroid carcinoma, in humans, because the human relevance of the rodent findings has not been determined [1].
The label names symptoms of thyroid tumors to watch for: a mass in the neck, dysphagia (trouble swallowing), dyspnea (trouble breathing), and persistent hoarseness [1]. It also states that routine monitoring of serum calcitonin or thyroid ultrasound is of uncertain value for early detection of medullary thyroid carcinoma in patients treated with Mounjaro [1].
Who should not take Mounjaro?
The label lists two contraindications. Mounjaro should not be used by people with a personal or family history of medullary thyroid carcinoma, or with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2) [1]. It also should not be used by anyone with a known serious hypersensitivity to tirzepatide or any of the product’s excipients [1].
Separately, the label states Mounjaro is not recommended in patients with severe gastroparesis [1].
Can Mounjaro cause pancreatitis?
Yes, acute pancreatitis is a labeled warning. In clinical studies, 14 events of acute pancreatitis were confirmed by adjudication in 13 Mounjaro-treated adult patients, a rate of 0.23 patients per 100 years of exposure, versus 3 events in 3 comparator-treated patients, or 0.11 per 100 years [1].
Postmarketing reports include hemorrhagic and necrotizing pancreatitis, sometimes fatal [1]. The label says to observe for signs and symptoms such as persistent or severe abdominal pain, sometimes radiating to the back, which may or may not come with nausea or vomiting, and to discontinue Mounjaro if pancreatitis is suspected [1].
In the UK, the MHRA updated product information for all GLP-1 and GLP-1/GIP receptor agonists, including tirzepatide, on 29 January 2026 to highlight the potential risk of severe acute pancreatitis, including rare necrotizing and fatal cases [7]. Between 2007 and October 2025, the MHRA received 1,296 Yellow Card reports of pancreatitis associated with these drugs, of which 19 were fatal and 24 were reported as necrotizing pancreatitis [7]. These are spontaneous reports, not incidence rates [7].
Does Mounjaro cause low blood sugar?
By itself, Mounjaro caused little hypoglycemia in trials. In a 40-week monotherapy trial, blood glucose below 54 mg/dL occurred in 0% of patients at every Mounjaro dose versus 1% on placebo, with no severe hypoglycemia in either group [1].
The risk changes when Mounjaro is combined with other diabetes drugs. Added to basal insulin over 40 weeks, glucose below 54 mg/dL occurred in 16%, 19%, and 14% at 5, 10, and 15 mg versus 13% on placebo, and severe hypoglycemia occurred in 0%, 2%, and 1% versus 0% on placebo [1]. Used with a sulfonylurea in a trial of up to 104 weeks, glucose below 54 mg/dL occurred in 13.8%, 9.9%, and 12.8%, and severe hypoglycemia in 0.5%, 0%, and 0.6% at 5, 10, and 15 mg [1].
The label states that the risk of hypoglycemia may be lowered by a reduction in the dose of the sulfonylurea or insulin, a decision that belongs to the prescriber [1].
What are the kidney, gallbladder, and severe GI warnings?
The label reports postmarketing cases of acute kidney injury, some requiring hemodialysis, with the majority occurring in patients who had GI reactions leading to dehydration, such as nausea, vomiting, or diarrhea [1]. No dosage adjustment is recommended for renal impairment, including end-stage renal disease, but the label says to monitor renal function in patients with renal impairment who report severe GI reactions [1].
Severe GI adverse reactions occurred in 1.3% at 5 mg, 0.4% at 10 mg, and 1.2% at 15 mg, versus 0.9% on placebo, in the pool of placebo-controlled trials; this warning section was added in December 2025 [1]. Postmarketing GI reports include ileus, intestinal obstruction, and severe constipation including fecal impaction [1].
Acute gallbladder disease (cholelithiasis, biliary colic, and cholecystectomy) was reported by 0.6% of Mounjaro-treated patients and 0% of placebo patients in the placebo-controlled trials [1].
What about eye effects, anesthesia, and pen sharing?
The label warns that temporary worsening of diabetic retinopathy has been reported with rapid improvement in glucose control, and that patients with a history of diabetic retinopathy should be monitored for progression; this warning was updated in August 2026 based on SURPASS-CVOT [1][4]. At baseline, 13% of the two-trial pool and 15% of the seven-trial pool had retinopathy [1].
The label also describes rare postmarketing reports of pulmonary aspiration in patients receiving GLP-1 receptor agonists during anesthesia or deep sedation, with residual gastric contents despite fasting, and states that data are insufficient on whether pausing the drug reduces risk; patients are instructed to tell healthcare providers about planned surgeries or procedures [1]. Finally, a Mounjaro KwikPen must never be shared between patients, even with a new needle, because of the risk of blood-borne pathogen transmission; that warning was added in January 2026 [1].
What other reactions and lab changes does the label report?
Injection site reactions occurred in 3.2% of Mounjaro-treated patients versus 0.4% on placebo in the placebo-controlled pool; in the seven-trial pool, they occurred in 4.6% of patients who developed anti-tirzepatide antibodies versus 0.7% of those who did not [1]. Hypersensitivity reactions (such as urticaria and eczema) occurred in 3.2% versus 1.7% on placebo, and in 4.1% of patients with antibodies versus 3.0% without in the seven-trial pool [1].
Dysesthesia was reported by 0.4% of patients at each dose and by no placebo patients; dysgeusia (altered taste) occurred in 0.1% versus 0% [1]. Treatment produced a mean heart rate increase of 2 to 4 beats per minute versus 1 beat per minute on placebo, and mean increases in pancreatic amylase of 33% to 38% and lipase of 31% to 42%; the label states the clinical significance of the enzyme elevations is unknown in the absence of other signs and symptoms of pancreatitis [1].
Does Mounjaro affect birth control or pregnancy?
Mounjaro delays gastric emptying, which may reduce the efficacy of oral hormonal contraceptives, with the largest effect after the first dose [1]. The label advises females using oral contraceptives to switch to a non-oral method or add a barrier method for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase [1].
On pregnancy, the label states that based on animal studies, Mounjaro may cause fetal harm, and that it should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus [1].
What did the long-term cardiovascular trial find?
SURPASS-CVOT enrolled 13,299 patients with type 2 diabetes and established atherosclerotic cardiovascular disease, randomized to tirzepatide up to 15 mg or dulaglutide 1.5 mg, double-blind, with a median follow-up of 210.1 weeks; 82% of patients were White [1][5].
The incidence of adverse events appeared similar between the two groups, though more gastrointestinal events were observed with tirzepatide [5]. Some reaction rates in this trial differed from the pooled trials: decreased appetite 17%, constipation 13%, vomiting 12%, abdominal pain 10%, fatigue 10%, and dysesthesia 0.7% [1].
For the composite of cardiovascular death, non-fatal heart attack, or non-fatal stroke, the rate was 12.2% on tirzepatide versus 13.1% on dulaglutide, a hazard ratio of 0.92 (95.3% CI 0.83 to 1.01); tirzepatide was noninferior to dulaglutide, and superiority was not established [1][5].
What about side effects in children 10 and older?
The pediatric indication was added in December 2025, with a maximum dosage of 10 mg once weekly in pediatric patients versus 15 mg in adults [1]. Safety data come from a 30-week placebo-controlled trial with a 22-week open-label extension in 99 pediatric patients [1].
During the 30-week placebo-controlled period, vomiting occurred in 3%, 16%, and 12% and abdominal pain in 9%, 22%, and 15% of patients on placebo, 5 mg, and 10 mg, respectively [1]. Blood glucose below 54 mg/dL occurred in 10%, 30%, and 27% of patients on placebo, 5 mg, and 10 mg when Mounjaro was added to basal insulin, but those percentages come from groups of only 10 to 11 patients each; with metformin alone the rates were 4%, 9%, and 9% [1]. No severe hypoglycemia was reported in the pediatric trial [1].
Are Mounjaro and Zepbound side effects the same?
Both products contain tirzepatide and share the same boxed warning on thyroid C-cell tumors and the same contraindications [1][2]. Their common-reaction lists overlap heavily but are not identical: Zepbound’s list of reactions reported in 5% or more of patients includes nausea, diarrhea, vomiting, constipation, abdominal pain, dyspepsia, injection site reactions, fatigue, hypersensitivity reactions, eructation, hair loss, and gastroesophageal reflux disease [2]. On the Mounjaro label, alopecia (hair loss) appears under postmarketing reports, where frequency cannot be reliably estimated [1].
The Zepbound label removed its warning on suicidal behavior and ideation in February 2026 [2]. The Mounjaro label cautions that adverse reaction rates from the clinical trials of one drug cannot be directly compared with rates from the trials of another drug [1], so the two lists should not be read as ranking one product against the other.
In the EU, the CHMP concluded on 12 December 2024 that a causal relationship between tirzepatide and dysaesthesia (the EU spelling of dysesthesia), and between tirzepatide and delayed gastric emptying, was at least a reasonable possibility, and amended the product information accordingly; the benefit-risk balance was unchanged [8].
Sources
- National Library of Medicine DailyMed. MOUNJARO and MOUNJARO KWIKPEN (tirzepatide) injection, solution. Drug label.
- National Library of Medicine DailyMed. ZEPBOUND and ZEPBOUND KWIKPEN (tirzepatide) injection, solution. Drug label.
- US Food and Drug Administration. MOUNJARO (tirzepatide) injection prescribing information. 2026.
- US Food and Drug Administration. Supplement approval letter for Mounjaro (tirzepatide) injection, NDA 215866/S-044 and S-045. 2026.
- New England Journal of Medicine. Cardiovascular outcomes with tirzepatide versus dulaglutide in type 2 diabetes.
- ClinicalTrials.gov. A study to evaluate tirzepatide (LY3298176) in pediatric and adolescent participants with type 2 diabetes mellitus inadequately controlled with metformin or basal insulin or both. NCT05260021.
- GOV.UK. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases. Drug safety update.
- European Medicines Agency. Mounjaro (tirzepatide): scientific conclusions and grounds for the variation to the terms of the marketing authorisation. December 12, 2024.
- GOV.UK. MHRA updates guidance for GLP-1 prescribers and patients. Press release.
Frequently asked questions
What side effects are most likely in the first week of Mounjaro?
The label's recommended starting dosage is 2.5 mg once weekly, with increases in 2.5 mg steps after at least 4 weeks on the current dose. The label states this escalation schedule is intended to reduce the risk of GI reactions, and that the majority of nausea, vomiting, and diarrhea reports occurred during dose escalation and decreased over time.
How long does Mounjaro nausea last?
The label states that the majority of nausea, vomiting, and diarrhea reports occurred during dose escalation and decreased over time. Discontinuation due to GI reactions occurred in 3.0% at 5 mg, 5.4% at 10 mg, and 6.6% at 15 mg versus 0.4% on placebo.
Does Mounjaro cause hair loss?
The Mounjaro label lists alopecia under postmarketing reports, where frequency cannot be reliably estimated from voluntary reporting. Zepbound, the same molecule approved for weight loss, lists hair loss among reactions reported in 5% or more of treated patients.
Who should not take Mounjaro?
The label contraindicates Mounjaro for anyone with a personal or family history of medullary thyroid carcinoma, anyone with MEN 2, and anyone with known serious hypersensitivity to tirzepatide or its excipients. It is also not recommended in patients with severe gastroparesis.
Does Mounjaro affect birth control pills?
Yes. Mounjaro delays gastric emptying, which may reduce the efficacy of oral hormonal contraceptives, with the largest effect after the first dose. The label advises switching to a non-oral method or adding a barrier method for 4 weeks after initiation and 4 weeks after each dose increase.
Can Mounjaro cause pancreatitis?
Yes, it is a labeled warning. Adjudicated clinical study events totaled 14 in 13 Mounjaro-treated patients, 0.23 per 100 patient-years, versus 0.11 for comparators. Postmarketing reports include hemorrhagic and necrotizing pancreatitis, sometimes fatal, and the label says to discontinue if pancreatitis is suspected.
Does Mounjaro cause low blood sugar?
As monotherapy over 40 weeks, glucose below 54 mg/dL occurred in 0% at every dose versus 1% on placebo, with no severe episodes. Combined with basal insulin, rates rose to 16%, 19%, and 14% at 5, 10, and 15 mg, and the label says reducing the insulin or secretagogue dose may lower the risk.
Are Mounjaro and Zepbound side effects the same?
Both contain tirzepatide and share the same boxed warning and contraindications. Their common-reaction lists overlap but differ; Zepbound's includes fatigue, injection site reactions, and hair loss at 5% or more. The Mounjaro label cautions that trial rates across different drugs cannot be directly compared.
This article summarizes FDA labeling, published research and company information current as of September 14, 2026. It is not medical advice and does not replace a conversation with your own healthcare provider. How we research and verify.