Tirzepatide: the monograph

In the only published head-to-head obesity trial (SURMOUNT-5, 751 adults, 72 weeks), tirzepatide reduced body weight by 20.2% on average versus 13.7% with semaglutide 2.4 mg, and more participants reached 10 to 25% loss thresholds. Both drugs' most common side effects were gastrointestinal.

SURMOUNT-5 is the direct comparison: 751 adults with obesity and no diabetes were randomized to the maximum tolerated dose of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg) for 72 weeks. Tirzepatide reduced weight by a least-squares mean of 20.2% versus 13.7% with semaglutide, cut waist circumference by 18.4 versus 13.0 cm, and more participants achieved reductions of at least 10%, 15%, 20%, and 25%. It was superior on the primary endpoint (P<0.001). In type 2 diabetes, SURPASS-2 found tirzepatide noninferior and superior to semaglutide 1 mg on HbA1c. Two honest caveats: SURPASS-2 used the 1 mg diabetes dose of semaglutide, and semaglutide 2.4 mg has a cardiovascular indication backed by the SELECT trial that tirzepatide does not yet have.

In SURMOUNT-1 (2,539 adults, 72 weeks), average weight loss was 15.0% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg, versus 3.1% with placebo. 85 to 91% of participants lost at least 5%; over half on the two higher doses lost 20% or more. With type 2 diabetes (SURMOUNT-2), averages were 12.8 to 14.7%.

The pivotal numbers: SURMOUNT-1 randomized 2,539 adults with obesity or overweight (no diabetes) to tirzepatide or placebo for 72 weeks. Mean weight change was -15.0% (5 mg), -19.5% (10 mg), and -20.9% (15 mg) versus -3.1% with placebo. 85%, 89%, and 91% of participants by dose lost at least 5% of body weight versus 35% on placebo, and 50% and 57% of the 10 and 15 mg groups lost 20% or more versus 3% on placebo. People with type 2 diabetes lose somewhat less on average: in SURMOUNT-2 the means were -12.8% (10 mg) and -14.7% (15 mg) versus -3.2%. After a lifestyle-program start (SURMOUNT-3), tirzepatide added a further 18.4% reduction while placebo participants regained 2.5%. Averages hide spread: individual results ranged widely in every trial.

Tirzepatide activates two incretin receptors, GIP and GLP-1 (semaglutide activates GLP-1 only). Per the FDA label, these receptors sit in appetite-regulating brain areas; the drug reduces appetite and food intake, boosts insulin only when glucose is high, lowers glucagon, and slows stomach emptying.

Tirzepatide is the first approved dual agonist of the GIP and GLP-1 receptors, the targets of two natural gut incretin hormones. The FDA label describes the mechanism: GLP-1 physiologically regulates appetite and caloric intake, both receptor types are present in appetite-regulating regions of the brain, and nonclinical studies suggest GIP contributes additional food-intake regulation. In type 2 diabetes it enhances first- and second-phase insulin secretion and reduces glucagon, both glucose-dependently, which is why hypoglycemia was not a feature of monotherapy trials. A C20 fatty diacid chain binds albumin and stretches the half-life to about 5 days, enabling once-weekly injection. The molecule was engineered from the GIP peptide backbone as Lilly compound LY3298176.

Gastrointestinal, led by nausea (25 to 29% vs 8% placebo), diarrhea (19 to 23% vs 8%), vomiting (8 to 13% vs 2%), and constipation (11 to 17% vs 5%). Most occur during dose escalation and fade. About 5 to 7% of trial patients stopped due to side effects vs 3.4% on placebo.

The label's most-common list (5% or more of patients): nausea, diarrhea, vomiting, constipation, abdominal pain, dyspepsia, injection site reactions, fatigue, hypersensitivity reactions, burping, hair loss, and gastroesophageal reflux disease. Pooled 72-week trial rates versus placebo: nausea 25 to 29% versus 8%, diarrhea 19 to 23% versus 8%, vomiting 8 to 13% versus 2%, constipation 11 to 17% versus 5%. The label notes most nausea, vomiting, and diarrhea occurred during dose escalation and decreased over time, which is why the schedule climbs in 4-week steps. Discontinuation for adverse reactions ran 4.8 to 6.7% by dose versus 3.4% with placebo. Hair loss tracked the weight reduction itself and was mostly reported by women. Serious but rarer risks (pancreatitis, gallbladder disease, kidney injury from dehydration, hypoglycemia with insulin or sulfonylureas) have their own section above.

Because rats given tirzepatide developed thyroid C-cell tumors in dose- and duration-dependent fashion. Whether that translates to humans is unknown, per the FDA label. As a precaution, anyone with a personal or family history of medullary thyroid carcinoma or MEN 2 must not use it.

The boxed warning rests on animal data: in a 2-year rat study, tirzepatide caused dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures. The label states plainly that it is unknown whether tirzepatide causes C-cell tumors, including medullary thyroid carcinoma, in humans, because the human relevance of the rodent finding has not been determined. The practical consequences: tirzepatide is contraindicated for anyone with a personal or family history of medullary thyroid carcinoma or with Multiple Endocrine Neoplasia syndrome type 2, and patients are told to report a neck mass, trouble swallowing, shortness of breath, or persistent hoarseness. Routine calcitonin or ultrasound monitoring is described by the label as of uncertain value. We grade this claim as animal evidence because that is what it is.

No. Compounded drugs are not FDA approved: FDA does not review them for safety, effectiveness, or quality before marketing. FDA says compounded drugs belong only where an approved drug cannot meet the medical need, and has warned about compounded GLP-1 shipments arriving warm or degraded.

Mounjaro and Zepbound are the FDA-approved tirzepatide products. A compounded version, whatever its source, is by definition not the approved product: FDA does not review compounded drugs for safety, effectiveness, or quality before they are marketed. FDA's stated position is that compounded drugs should be used only when a patient's medical need cannot be met by an FDA-approved drug, with a prescription filled at a state-licensed pharmacy. The agency has also documented practical problems in the compounded GLP-1 market: complaints of injectables arriving warm with inadequate ice packs (its advice: do not use them), telehealth red flags such as claims that a compounded drug is the same as the approved one, and border actions against ingredient suppliers with quality concerns. Tirzepatide is FDA-approved as Mounjaro (type 2 diabetes) and Zepbound (chronic weight management). Compounded or 'research' tirzepatide is not the FDA-approved product.

SURMOUNT-4 measured exactly this: after 36 weeks on tirzepatide (average 20.9% loss), people switched to placebo regained 14.0% over the next year, while those who continued lost another 5.5%. At week 88 the gap was -25.3% versus -9.9% from start.

SURMOUNT-4 was a randomized withdrawal trial built to answer this question. All 670 participants took tirzepatide for a 36-week lead-in and averaged a 20.9% weight reduction. Then half were blindly switched to placebo: over the following 52 weeks they regained a mean 14.0% of body weight, while participants who stayed on tirzepatide lost a further 5.5%. Measured from day zero to week 88, continued treatment reached -25.3% versus -9.9% for those withdrawn. 89.5% of continuing participants kept at least 80% of their lead-in loss, versus 16.6% of those switched. The trial's conclusion, in the authors' words, was that withdrawal led to substantial regain while continued treatment maintained and augmented the reduction. This mirrors the label's framing of obesity as requiring long-term management. Decisions about stopping belong with the prescribing clinician.

Trial curves flatten late: most loss accrues across the first year-plus, then stabilizes. The 176-week SURMOUNT-1 extension showed reductions held near 19.7% (15 mg) at 3 years, and SURMOUNT-4 showed continued treatment after week 36 still added 5.5% more loss. A plateau is stability, not failure.

Randomized data bracket the plateau question in two ways. Duration: in the SURMOUNT-1 extension, participants on 15 mg averaged -20.9% at 72 weeks and -19.7% at 176 weeks, meaning the reduction held essentially flat through 3 years of continued dosing rather than continuing to fall or rebounding. Continued gains after the escalation phase: in SURMOUNT-4, people already 36 weeks in (at -20.9%) who kept taking tirzepatide lost a further 5.5% by week 88, so loss continued well past the first 9 months before leveling. The honest read: on average, weight declines through roughly the first 12 to 18 months, then holds steady while treatment continues, and individual timing varies. The alternative to a plateau is documented too: those switched to placebo at week 36 regained 14.0%.

Start at 2.5 mg once weekly for 4 weeks, then 5 mg. If needed, raise in 2.5 mg steps no faster than every 4 weeks. Maintenance for weight management is 5, 10, or 15 mg weekly (sleep apnea: 10 or 15 mg). Maximum is 15 mg once weekly.

The label's escalation schedule applies to all indications: begin at 2.5 mg injected subcutaneously once weekly for 4 weeks (an initiation dose, not maintenance), then move to 5 mg. Further increases go in 2.5 mg increments with at least 4 weeks on the current dose, expressly to reduce gastrointestinal side effects. Recommended maintenance for weight reduction is 5 mg, 10 mg, or 15 mg once weekly based on response and tolerability; for obstructive sleep apnea it is 10 mg or 15 mg; for type 2 diabetes, Mounjaro escalates as additional glycemic control is needed (maximum 10 mg weekly in pediatric patients). The overall maximum is 15 mg once weekly. If a maintenance dose is not tolerated, the label allows stepping down. Injections go in the abdomen, thigh, or back of the upper arm, any time of day, with or without food.

Label rule: take it as soon as possible within 4 days (96 hours) of the missed dose. If more than 4 days have passed, skip it and take the next dose on the usual day. Never double up. You can move your weekly day if doses stay at least 72 hours apart.

The label gives a precise 2-part rule. Within 4 days (96 hours) of the missed dose: inject as soon as possible, then resume the regular weekly schedule. More than 4 days later: skip the missed dose entirely and take the next one on the regularly scheduled day. The label also permits changing the weekly injection day when needed, provided at least 3 days (72 hours) separate two doses. The 4-day window tracks the drug's pharmacology: with a half-life of about 5 days, levels decline slowly enough that a mid-week catch-up keeps exposure in range. For comparison, semaglutide's window is tighter: Wegovy's label says to skip unless the next dose is more than 48 hours away.

Yes. On December 20, 2024, FDA approved Zepbound for moderate to severe obstructive sleep apnea in adults with obesity, the first medication approved for OSA. In the 52-week trials it cut the apnea-hypopnea index by 25 to 29 events per hour versus about 5 with placebo.

FDA approved Zepbound for moderate to severe obstructive sleep apnea in adults with obesity on December 20, 2024, calling it the first drug treatment option for OSA. The evidence came from two 52-week randomized, placebo-controlled trials in 469 adults: one in people not using positive airway pressure and one in people on PAP therapy. Tirzepatide reduced the apnea-hypopnea index by 25.3 events per hour versus 5.3 with placebo in the first trial and 29.3 versus 5.5 in the second, treatment differences of 20.0 and 23.8 events per hour. Secondary outcomes also improved: body weight, hypoxic burden, hsCRP, systolic blood pressure, and patient-reported sleep measures. The OSA maintenance dosage is 10 mg or 15 mg weekly. FDA's announcement attributes the AHI improvement primarily to weight reduction.

Across the SURPASS trials, HbA1c fell 1.9 to 2.6 percentage points depending on dose and comparator, beating semaglutide 1 mg, insulin degludec, and insulin glargine in head-to-head trials. Monotherapy caused no clinically significant hypoglycemia.

Mounjaro's approval rests on the SURPASS program. As monotherapy (SURPASS-1), HbA1c fell 1.87 to 2.07 percentage points versus a slight rise on placebo, with no clinically significant hypoglycemia. Against semaglutide 1 mg (SURPASS-2), reductions were 2.01 to 2.30 points versus 1.86, noninferior and superior. Against titrated insulin degludec (SURPASS-3): 1.93 to 2.37 points versus 1.34. Against insulin glargine in high-cardiovascular-risk patients (SURPASS-4): 2.43 to 2.58 points versus 1.44, with far less hypoglycemia (6 to 9% versus 19%). Added to insulin glargine (SURPASS-5): 2.11 to 2.40 points versus 0.86 with placebo. Weight fell in every trial as well. Mounjaro is indicated as an adjunct to diet and exercise in adults and children 10 and older with type 2 diabetes.

No: the label says weight loss offers no benefit in pregnancy and may cause fetal harm, and to discontinue when pregnancy is recognized. Animal studies showed fetal harm. Oral contraceptives also absorb less reliably for 4 weeks after starting and after each dose increase.

The label advises against use in pregnancy on two grounds: weight loss itself offers no benefit to a pregnant patient and may cause fetal harm, and animal reproduction studies in rats and rabbits showed fetal growth reductions and abnormalities at clinically relevant exposures. Human data are insufficient to establish a drug-associated risk estimate. The instruction is to discontinue Zepbound when a pregnancy is recognized, and a pregnancy exposure registry (1-800-545-5979) tracks outcomes. There is a related contraception interaction: because tirzepatide delays gastric emptying, the label tells patients on oral hormonal contraceptives to switch to a non-oral method or add a barrier method for 4 weeks after initiation and for 4 weeks after each dose escalation. Anyone planning pregnancy should raise timing with their clinician.