Tirzepatide

What is tirzepatide?

Tirzepatide is a once-weekly injectable drug developed by Eli Lilly that activates two gut hormone receptors at once: the GLP-1 receptor and the GIP receptor. It is a synthetic peptide of 39 amino acids, and it was the first dual agonist of this kind to reach the market.

The FDA approved it in 2022 as Mounjaro for type 2 diabetes and in 2023 as Zepbound for chronic weight management in adults with obesity, or overweight plus a weight-related condition. Zepbound was later also approved for moderate to severe obstructive sleep apnea in adults with obesity.

Tirzepatide sits at the center of the current wave of obesity medicine, alongside semaglutide. Unlike most compounds in this directory, it has a large, high-quality evidence base from industry-funded phase 3 trials involving thousands of participants.

How tirzepatide works

After you eat, your gut releases hormones called incretins. Two of the most important are GLP-1 and GIP. They tell the pancreas to release insulin when blood sugar rises, slow the rate at which the stomach empties, and signal the brain that you are full. Tirzepatide imitates both hormones, but lasts for days instead of minutes. The result is lower appetite, smaller meals, better blood sugar control and, over months, substantial weight loss.

Why add GIP? GLP-1 drugs alone already cause weight loss. GIP receptor activity appears to add to the effect through actions in fat tissue and the brain, and it may soften some of the nausea associated with GLP-1 activity. The precise contribution of GIP is still debated.

In more precise terms, tirzepatide is a 39-residue peptide based on the native GIP sequence, engineered to bind both the GIP receptor and the GLP-1 receptor, with greater relative activity at the GIP receptor. A C20 fatty diacid side chain attached through a linker lets it bind albumin in the blood, which slows clearance and gives a half-life of about five days. That allows once-weekly dosing. Doses are increased gradually over about 20 weeks to limit gastrointestinal side effects.²

What the research shows on tirzepatide

Weight loss is the headline result. In SURMOUNT-1, 2,539 adults with obesity but without diabetes were randomized to tirzepatide 5, 10 or 15 mg weekly or placebo for 72 weeks. Average weight loss was 15.0%, 19.5% and 20.9% with the three doses, compared with 3.1% on placebo. About half of participants on the two higher doses lost at least 20% of their body weight.²

Longer follow-up in participants who also had prediabetes showed weight loss of 12.3% to 19.7% at three years, against 1.3% on placebo. Only 1.3% of tirzepatide recipients developed type 2 diabetes, compared with 13.3% on placebo.¹

Tirzepatide has now been compared directly with semaglutide twice. In type 2 diabetes, all three doses lowered HbA1c more than semaglutide 1 mg over 40 weeks, and produced more weight loss.⁸ In obesity without diabetes, SURMOUNT-5 found average weight loss of 20.2% with tirzepatide versus 13.7% with semaglutide at 72 weeks.⁶

Benefits extend beyond the scale. In obstructive sleep apnea, tirzepatide reduced breathing interruptions by about 25 to 29 events per hour, compared with about 5 on placebo.⁴ In patients with obesity and heart failure with preserved ejection fraction, it lowered the combined risk of cardiovascular death or worsening heart failure and improved symptoms.⁵

There are important limits. When participants who had lost weight on tirzepatide were switched to placebo, they regained about 14% of their body weight within a year, while those who continued kept losing.³ And not all lost weight is fat. In a body composition substudy, about a quarter of the weight lost was lean mass, the same proportion seen with placebo.⁷ All of these trials were funded by Eli Lilly.

Studied for: Type 2 diabetes, Chronic weight management, Obstructive sleep apnoea with obesity.

Tirzepatide reference data

Tirzepatide dosage in published research

How tirzepatide is used in practice

Tirzepatide is prescribed by primary care doctors, endocrinologists, obesity specialists and many telehealth services. It is injected under the skin once a week, starting at a low dose and stepping up every four weeks as tolerated. Treatment is intended to be long term, since stopping usually leads to weight regain.

Cost and insurance coverage shape access. Coverage for diabetes is common, while coverage for weight loss varies widely. Lilly also sells Zepbound vials directly to self-paying patients at a lower price than the pens.

During the drug shortage of 2022 to 2024, compounding pharmacies were allowed to make copies of tirzepatide, and many telehealth companies sold them. After the FDA declared the shortage over, that allowance ended. Some compounded and research-chemical versions are still sold. These products are not FDA reviewed, and the FDA has received reports of dosing errors and adverse events linked to compounded GLP-1 drugs.

Tirzepatide risks and side effects

The most common side effects are gastrointestinal: nausea, diarrhea, vomiting, constipation and indigestion. They are usually mild to moderate and occur mainly while the dose is being increased.² In SURMOUNT-1, about 4% to 7% of participants on tirzepatide stopped because of side effects.

Less common but more serious risks include pancreatitis, gallbladder disease, dehydration leading to kidney injury, and severe allergic reactions. Low blood sugar is uncommon on tirzepatide alone but becomes more likely with insulin or sulfonylureas. Because it slows stomach emptying, anesthesiologists may need to know about it before procedures.

Tirzepatide carries a boxed warning about thyroid C-cell tumors seen in rodents. It should not be used by people with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia type 2. It is not recommended in pregnancy. It can reduce the effectiveness of oral contraceptives, so the label advises a non-oral or backup method for four weeks after starting and after each dose increase.

Loss of lean mass during rapid weight loss is a concern for older adults, which is why resistance training and adequate protein are often advised alongside treatment.⁷

Reported side effects

Alternatives to tirzepatide

Semaglutide (Wegovy for weight, Ozempic and Rybelsus for diabetes) is the closest alternative and has its own cardiovascular outcome data, though it produced less weight loss than tirzepatide in a head-to-head trial.⁶ Liraglutide is an older daily GLP-1 drug with smaller effects.

Other approved weight-loss medicines include phentermine-topiramate and naltrexone-bupropion, which are oral but generally less effective. Bariatric surgery still produces the largest and most durable weight loss for people with severe obesity. For sleep apnea, positive airway pressure therapy remains standard, and tirzepatide was tested both with and without it.⁴

Questions for a clinician

• What evidence applies to my situation, and at what level?
• What established alternatives should I consider first?
• What monitoring would be necessary, and how often?