Wegovy vs Mounjaro Side Effects: A Head-to-Head Comparison

Is Mounjaro a weight-loss drug?

Mounjaro is approved for type 2 diabetes and cardiovascular risk reduction in certain adults with type 2 diabetes; it is not labeled for weight loss [4].

That matters for a side-effect comparison. Wegovy's safety figures come from weight-management trials in adults with obesity or overweight, while Mounjaro's come from trials in adults with type 2 diabetes [3][4].

SURMOUNT-5 compared tirzepatide with semaglutide at weight-management doses. It randomized 751 adults with obesity and no diabetes to maximum tolerated tirzepatide (10 or 15 mg) or maximum tolerated semaglutide (1.7 or 2.4 mg) for 72 weeks. In that trial, stomach-related side effects were the most common in both groups, and most were mild to moderate [1]. The trial's reported result was weight change: -20.2% with tirzepatide vs -13.7% with semaglutide, and waist circumference -18.4 cm vs -13.0 cm [1].

What both labels warn about

The boxed warning is the same on each: in rodents the drugs caused dose- and duration-dependent thyroid C-cell tumors at clinically relevant exposures, human relevance is unknown, and both are contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2 [4][3]. Both labels say routine calcitonin monitoring or thyroid ultrasound is of uncertain value [4][3].

Both also warn about acute pancreatitis, with instructions to stop the drug if it is suspected; gallbladder disease; acute kidney injury from volume depletion; hypoglycemia when combined with insulin or a sulfonylurea; serious hypersensitivity including anaphylaxis and angioedema; and pulmonary aspiration during general anesthesia or deep sedation. Both say the drug is not recommended in people with severe gastroparesis [4][3].

SURPASS-2: the head-to-head trial with rates

SURPASS-2 enrolled 1,879 adults with type 2 diabetes on metformin and compared tirzepatide 5, 10 or 15 mg with semaglutide 1 mg weekly for 40 weeks, open-label [2]. Gastrointestinal rates were close: the semaglutide arm used 1 mg, the diabetes dose rather than Wegovy's 2.4 mg weight-management dose, and nausea occurred in 17% to 22% with tirzepatide vs 18% with semaglutide, diarrhea 13% to 16% vs 12%, vomiting 6% to 10% vs 8% [2][3].

Why the label percentages cannot be ranked

Taken alone, the two labels look far apart. In Wegovy's pooled weight-management trials (2,116 on Wegovy 2.4 mg, 1,261 on placebo, up to 68 weeks), nausea occurred in 44% vs 16%, diarrhea 30% vs 16%, vomiting 24% vs 6% and constipation 24% vs 11%; any GI reaction 73% vs 47%, and 6.8% vs 3.2% stopped treatment for an adverse reaction [3]. In Mounjaro's pooled type 2 diabetes trials (235 on placebo and roughly 240 on each dose, mean 36.6 weeks), nausea occurred in 18% at 15 mg vs 4% on placebo, diarrhea 17% vs 9%, vomiting 9% vs 2% and constipation 7% vs 1% [4].

The placebo groups also differ: 16% of placebo patients in the Wegovy trials reported nausea against 4% in the Mounjaro trials, and the Wegovy pool included 19% of patients with type 2 diabetes [3][4]. The Mounjaro label's own caution applies: rates from different trials cannot be directly compared [4].

Gallbladder, pancreas and thyroid risk in each label

Both labels warn about gallbladder disease. The Wegovy label gives cholelithiasis in 1.6% vs 0.7% on placebo and cholecystitis in 0.6% vs 0.2% in adult weight-management trials [3].

On pancreatitis, the Wegovy label reports four confirmed cases in Wegovy-treated adults, 0.2 per 100 patient-years, vs one on placebo [3]. A 2017 systematic review pooled three long-term placebo-controlled trials of GLP-1 receptor agonists in type 2 diabetes (9,347 treated patients, 9,353 on placebo) and found no increased risk of acute pancreatitis (Peto odds ratio 0.745, 95% CI 0.47 to 1.17), while noting that more evidence was needed [5].

For thyroid cancer in people, a nested case-control study in the French national health database compared 2,562 people with thyroid cancer to 45,184 matched controls. It found that one to three years of GLP-1 receptor agonist use was associated with a higher risk of all thyroid cancer (adjusted hazard ratio 1.58, 95% CI 1.27 to 1.95) and of medullary thyroid cancer (adjusted hazard ratio 1.78, 95% CI 1.04 to 3.05) [6]. That study covered GLP-1 receptor agonists as a group, in people with type 2 diabetes treated between 2006 and 2018, and is not specific to Wegovy or tirzepatide. It is an association in an observational database, not a demonstration of cause. The label's rodent finding likewise leaves the human relevance unknown [3][4].

Where the labels differ

Hair loss: the Wegovy label reports hair loss in 3% vs 1% on placebo [3]. The Mounjaro label's list of common adverse reactions — nausea, diarrhea, decreased appetite, vomiting, constipation, dyspepsia and abdominal pain — does not include hair loss [4]. That does not mean tirzepatide is free of this effect: the Zepbound label, which covers the same drug in weight-management trials, reports hair loss in 4% to 5% vs 1% on placebo, associated with weight reduction and more common in women (7.1% of women vs 0.5% of men).

Altered skin sensation: dysesthesia appears in the Wegovy label at 2% vs 1% on 2.4 mg, and at 22% vs 0% on the 7.2 mg dose, where 18% of those affected had not recovered by the end of the trial [3].

Heart rate: the Wegovy label reports a mean resting heart rate rise of 1 to 4 beats per minute, with a rise of 20 beats per minute or more in 26% vs 16% on placebo [3]. The tirzepatide label reports a mean rise of 1 to 3 beats per minute [4].

Oral contraception: the tirzepatide label advises people using oral hormonal contraceptives to switch to a non-oral method or add a barrier method for 4 weeks after starting and for 4 weeks after each dose increase, because of delayed gastric emptying [4]. The semaglutide labels carry no such advice, and a tablet interaction study found no clinically significant change in ethinyl estradiol or levonorgestrel [3].

What the labels say to look out for

The Mounjaro label describes the signs of acute pancreatitis as persistent or severe abdominal pain, sometimes radiating to the back, with or without nausea or vomiting [4]. Both labels counsel patients about symptoms of thyroid tumors, such as a mass in the neck, trouble swallowing, shortness of breath and persistent hoarseness [4][3]. The Mounjaro label also notes that most reports of nausea, vomiting and diarrhea happened during dose escalation and decreased over time [4].

The Wegovy label adds a limitation of use: Wegovy with any other GLP-1 receptor agonist is not recommended [3].