Tirzepatide (Mounjaro, Zepbound): what it is, how it works, results and side effects

What is tirzepatide?

Tirzepatide is the name of the medicine. Mounjaro, Zepbound and Yurpeak are brand names it is sold under.

Every medicine has two kinds of name. The medicine's own name is what works in your body. The brand is the name on the box, chosen by the company selling it. One medicine can have many brands.

Tirzepatide works the same way. Two brands can contain the same medicine and still differ in what they're approved for. In the US, Mounjaro's label covers type 2 diabetes, and Zepbound's covers weight and sleep apnoea.^1,2 In the European Union, the Mounjaro brand covers both diabetes and weight.³

Tirzepatide and some of its brands

Names only, not a recommendation. Brand names and approved uses differ by country, and this list isn't complete.

The US approved Mounjaro in May 2022 and Zepbound in November 2023.^25,26 Other GLP‑1 medicines include semaglutide, liraglutide and dulaglutide.

How does it work in your body?

It copies two hormones your gut releases after you eat: GLP‑1 and GIP. Most other medicines in this family, such as semaglutide, copy only GLP‑1.¹⁰

Hormones are chemical messengers. After a meal, your gut releases two that tell the body food has arrived:⁴

• GLP‑1 (glucagon-like peptide‑1), from the lower gut.⁴
• GIP (glucose-dependent insulinotropic polypeptide), from the upper gut. Both help the pancreas release insulin when blood sugar is high.⁴

Tirzepatide switches on the body's receptors (sensors) for both hormones.¹ That's why it's called a "dual" medicine. A fatty chain added to the molecule makes it last much longer than your own hormones.¹

Because the insulin and glucagon effects only switch on when sugar is high, tirzepatide on its own rarely drops sugar too low. In a diabetes trial of tirzepatide alone, nobody on it had clinically significant low blood sugar.⁹ The risk goes up when it's taken with insulin or certain diabetes pills (see serious risks).¹

Why once a week, and how long it stays in your body

Your body clears tirzepatide slowly. Its half-life, the time it takes for half a dose to leave the body, is about 5 days. That is what makes one injection a week enough.^1,3

The same slow clearing means it doesn't leave your body the day you stop. That's why the European label says to stop it at least 1 month before a planned pregnancy.³ It also means side effects can take a while to fade after a dose.

Who is it for?

Adults with type 2 diabetes, and adults with obesity, or overweight plus a weight-related health problem. A "label" is the official document that says what a medicine is approved for, and it differs by country.

• Type 2 diabetes: alongside diet and exercise, in adults and in children aged 10 and over (US Mounjaro label, and the European label).^1,3
• Weight management: alongside a reduced-calorie diet and more physical activity. The European label sets this at a BMI of 30 or more, or 27 to 30 with at least one weight-related condition, such as high blood pressure, high cholesterol, sleep apnoea, heart disease, prediabetes or type 2 diabetes.³ The US Zepbound label covers adults with obesity, or overweight with at least one weight-related condition.²
• Sleep apnoea: moderate to severe obstructive sleep apnoea in adults with obesity (US Zepbound label).²
• Heart risk: lowering the risk of heart attack, stroke and heart-related death in adults with type 2 diabetes at high risk of these (US Mounjaro label, added in August 2026).¹

BMI (body mass index) is your weight in kilograms divided by your height in metres, squared. It's a rough guide, not a diagnosis. Work out your own BMI.

Lifestyle changes come first, for everyone. Every label pairs tirzepatide with diet and activity, so the medicine is added to them, not used instead.^2,3 Surgery is for much higher BMIs.²¹

Many Asian countries use lower cut-offs. Health problems linked to weight start at a lower BMI in many Asian people. A 2022 surgery guideline says a BMI over 25 suggests obesity in Asian people.²¹ Tirzepatide's trial in China took people with a BMI of 28 or more, or 24 with a weight-related condition.¹⁶ For India's cut-offs, see the India guide on weight and health.

These are points where you and a doctor can start a conversation, not rules that say you qualify. A doctor also looks at your health, your other medicines and your history.

What "off-label" means

Using a medicine for something its label doesn't cover is called "off-label" use. One example: in the US, using the Mounjaro brand only for weight, when the weight brand there is Zepbound.^1,2

The rules differ by country. In the US, the FDA says that once a medicine is approved, doctors generally may prescribe it for an unapproved use when they judge it medically appropriate for their patient.²² Ask your doctor which use your prescription is for.

How do you take it?

One injection under the skin, once a week. You start low and go up slowly, which lowers the risk of stomach side effects.¹

1. Weeks 1–42.5 mg once a week. This is a starting dose only, not a dose to stay on.²
2. Week 5 on5 mg once a week.^2,3
3. Then, if neededUp by 2.5 mg at a time (7.5, 10, 12.5 mg), after at least 4 weeks on each dose.^1,3
4. Up to 15 mgThe highest dose for adults. The earliest you can reach it is week 21.¹

• Doses you stay on: 5, 10 or 15 mg for weight; 10 or 15 mg for sleep apnoea.² For children aged 10 and over with type 2 diabetes, the highest dose is 10 mg.^1,3
• If a dose is too hard to manage, the label says a doctor can consider a lower dose to stay on.²
• Any time of day, with or without food.¹
• Where: the belly or thigh. Someone else can inject the back of your upper arm. Use a different spot each week.¹
• Forms: single-dose pens, single-dose vials, multi-dose vials, and a multi-dose KwikPen holding 4 weekly doses. Which ones are sold depends on the country.¹ With a vial, you need a new syringe and needle for every injection.¹
• Not with other similar medicines. Don't combine it with another tirzepatide product or any GLP‑1 medicine.²

What did the trials find?

In the main weight trial, people lost about 15–21% of their body weight over 72 weeks, depending on dose. People with type 2 diabetes lost less.

The trials below are "randomised controlled trials". They work like this:

• People are split at random into groups. One group gets the medicine. Another gets a dummy injection with no medicine in it (a placebo).
• Everyone gets the same diet and activity advice, and nobody knows which injection they're on.
• So the gap between the groups is what the medicine itself added.⁵

The tirzepatide trials below were paid for by Eli Lilly, which makes it.^5,6,7

Weight

Average share of body weight lost. Kilograms are for someone who starts at 90 kg (198 lb).

Most people lost a meaningful amount, but not everyone. In SURMOUNT-1, 91 in 100 people on 15 mg lost at least 5% of their weight, against 35 in 100 on the dummy injection. Put the other way, about 9 in 100 on the top dose lost less than 5%. At the other end, 57 in 100 on 15 mg lost 20% or more, against 3 in 100 on the dummy injection.⁵

All the figures above are averages. Some people lose much more, and some lose very little.

In East Asian trials, results were similar.

• China (210 adults, SURMOUNT-CN, 52 weeks): 13.6% lost on 10 mg and 17.5% on 15 mg, against 2.3% on a dummy injection.¹⁶
• Japan (225 adults analysed, SURMOUNT-J, 72 weeks): people on 10 mg and 15 mg lost about 16% and 21% more of their body weight than people on a dummy injection.¹⁷

Against semaglutide, head to head

You can't compare separate trials, such as SURMOUNT-1 and semaglutide's STEP 1, because they had different people. SURMOUNT-5 tested the two medicines against each other in the same trial. Each person took the highest dose they could manage. Everyone knew which medicine they were on, and Lilly paid for it.⁷

Semaglutide has its own page: semaglutide (Ozempic, Wegovy, Rybelsus).

Blood sugar in type 2 diabetes (the SURPASS trials)

HbA1c is a blood test that shows your average blood sugar over the past 2 to 3 months. The diabetes trials of tirzepatide are called SURPASS, and they tested the Mounjaro doses.

How much HbA1c fell, in points

SURPASS-2 compared tirzepatide with a 1 mg semaglutide dose. Semaglutide is also given at higher doses, up to 2.4 mg in SURMOUNT-5 above, so this trial doesn't settle which lowers sugar more at every dose.^7,10

Sleep apnoea

In obstructive sleep apnoea, the airway keeps closing during sleep, so breathing stops or becomes very shallow again and again.¹³ Its severity is measured as the number of these pauses per hour of sleep, called the AHI.¹¹

In December 2024, the US approved Zepbound for moderate to severe sleep apnoea in adults with obesity.¹² Europe's regulator agreed it improves sleep apnoea, but said a separate approval wasn't needed because the weight approval already covers these people.¹³

The heart

SURPASS-CVOT compared tirzepatide (up to 15 mg) with dulaglutide (1.5 mg), an older weekly medicine already shown to lower heart events. It took 13,299 adults with type 2 diabetes and heart or blood-vessel disease, for about 4 years.^14,1

• A heart attack, stroke or heart-related death happened to 12.2 in 100 on tirzepatide, against 13.1 in 100 on dulaglutide.¹⁴
• That showed tirzepatide was not worse than dulaglutide. It did not prove it was better.^14,1
• Lilly paid for the trial.¹⁴ The US added the heart-risk use to Mounjaro's label in August 2026.¹

The dulaglutide page explains that medicine: dulaglutide (Trulicity).

What are the common side effects?

Stomach and gut effects. In the weight trials, 56 in 100 people on tirzepatide had at least one, against 30 in 100 on a dummy injection.²

People on the dummy injection get some stomach symptoms too. So the real effect of the medicine is the difference between the two numbers. Most of these effects were mild or moderate, and most nausea, vomiting and diarrhoea came during dose increases, then eased.^2,5

Out of every 100 people, how many felt sick (nausea) at some point:

Some people stop because of side effects. In the weight trials, 4 in 100 on 15 mg stopped because of stomach side effects, against fewer than 1 in 100 on the dummy injection. Most who stopped did so in the first few months.²

Side effects at a glance

How many people out of 100 had each one in the weight trials, on 5 mg and on 15 mg, against a dummy injection. The tag gives the frequency word from the European label.^2,3

Figures: the US Zepbound label, which pools two weight trials (SURMOUNT-1 and -2), up to 72 weeks. 10 mg results sat close to these.² Frequency words: the European Mounjaro leaflet, where "very common" means more than 1 in 10 people and "common" means 1 in 10 to 1 in 100.³ Some words apply only to people treated for weight.

What helps

• Eat smaller meals, and go easy on fatty and heavy food, especially after a dose step.¹⁹
• Drink enough fluids. Vomiting and diarrhoea can dehydrate you, and that can harm the kidneys.^2,19
• Slower dose steps are a doctor's call. Don't change your dose on your own.²

Muscle, and how to protect it

Some of the weight you lose isn't fat. In a scan study inside SURMOUNT-1, 160 people had body scans at the start and at week 72. Of the weight lost, about 75% was fat and 25% was lean mass, on tirzepatide and on the dummy injection alike.¹⁸ Lean mass means everything that isn't fat: muscle, but also organs, bone and water.

A 2025 joint advisory from four US obesity and nutrition societies gives this advice for people on these medicines:¹⁹

If you have kidney disease, ask your doctor before eating more protein.

Serious risks, and who shouldn't take it

Serious problems are uncommon or rare, but you should know the warning signs. Most come from the official warnings on the labels.

Who shouldn't take it

• A personal or family history of medullary thyroid cancer (a rare thyroid cancer), or MEN 2 (multiple endocrine neoplasia type 2, a rare inherited condition that causes gland tumours).¹
• A known serious allergy to tirzepatide or anything in the injection.¹
• Pregnancy. The European label says it isn't recommended in pregnancy, or in women who could get pregnant and aren't using contraception.³
• Severe gastroparesis (a stomach that empties very slowly): the US label says it isn't recommended.²

The numbers below compare people on tirzepatide with people on a dummy injection, where the trials give both. "Per 100 patient-years" means the number of cases if 100 people took it for a year.

The contraceptive pill, pregnancy and planning a baby

Tirzepatide can make the contraceptive pill less reliable. It slows the stomach, so the pill is absorbed less well. The effect is biggest after the first dose.¹ The labels word their advice differently:

• US label and the UK regulator (MHRA): switch to a method that isn't a pill, such as a coil or implant, or add a barrier method such as condoms. Do this for 4 weeks after starting, and for 4 weeks after every dose increase.^1,20
• European label: it judged the drop after one dose not clinically important, but recommends contraception for women who could get pregnant while on tirzepatide.³

Planning a pregnancy? The European label and the UK regulator say to stop tirzepatide at least 1 month before trying, because it leaves the body slowly.^3,20 The US label says to stop it once a pregnancy is known: weight loss offers no benefit in pregnancy and may harm the baby.² Talk to your doctor before you stop, especially if you have diabetes.

Fake and unapproved products

• Sold without a prescriptionIn the UK and India, for example, it is prescription-only.^20,23
  A doctor assesses you first, and a licensed pharmacy supplies it against a prescription.
• A shared penThe US label says never to share a KwikPen, even with a new needle.¹
  Your own pen or vial, used only by you, with a new needle each time.¹

What happens when you stop?

For most people, much of the weight comes back. Tirzepatide treats weight while you take it; it doesn't cure it.

SURMOUNT-4 tested this directly. Everyone took tirzepatide for 36 weeks first and lost 20.9% of their weight on average. Then half were switched, without knowing, to a dummy injection for another 52 weeks.¹⁵

If you have type 2 diabetes, your blood sugar can rise again when you stop.³ Don't stop without talking to your doctor, who can plan it with you.³

Missed doses, storage and pens

You have a 4-day window for a missed dose. Keep pens in the fridge until you start them.

If you miss a dose (US and European labels):^1,3

• 4 days (96 hours) or less since it was due: take it as soon as you remember, then go back to your usual day.
• More than 4 days: skip it, and take the next one on your usual day.
• Never take two doses to make up. Doses must be at least 3 days apart.
• You can change your injection day, as long as the two doses are at least 3 days apart.

Storage (US label; check the leaflet in your own pack):¹

• Keep unused pens and vials in the fridge, at 2–8 °C (36–46 °F), in the box, away from light.
• Never freeze it, and don't use it if it has frozen.
• Single-dose pens and vials can stay out of the fridge, below 30 °C (86 °F), for up to 21 days in total.
• An opened KwikPen can be kept in the fridge or below 30 °C. Throw it away after 30 days, or after 4 weekly doses, even if some is left.

Questions people ask

The other medicines in this family