How Does Mounjaro Work, and How Long Does It Take to Work?

Mounjaro is a once-weekly injection that copies two gut hormones, GIP and GLP-1. It works mainly by reducing hunger and helping you feel full sooner. This guide explains what it does in the body, how quickly the effects start, and what the main weight management trial found over time, using the UK product information and the published trial.

How Mounjaro Works: The Short Answer

It acts on GIP and GLP-1 receptors and lowers weight mainly by reducing hunger and how much people eat.

Mounjaro contains tirzepatide, which acts on two hormone receptors in the body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). These are hormones the gut releases after eating.

According to its Summary of Product Characteristics (SmPC), tirzepatide lowers body weight mainly by reducing how much people eat: it increases feelings of fullness, decreases hunger and reduces food cravings. In people with type 2 diabetes it also lowers blood sugar. It is licensed in the UK for weight management, alongside a reduced-calorie diet and more physical activity, and for type 2 diabetes.

What It Does in the Body

It acts on appetite areas of the brain, helps control blood sugar and slows stomach emptying.

The SmPC describes several effects:

• Appetite: both GIP and GLP-1 receptors are found in areas of the brain important to appetite. In clinical studies, people on tirzepatide ate less at meals, felt less hungry and had fewer cravings for fatty foods, sweets and starchy foods.

• Fat: GIP receptors are also found on fat cells, and the SmPC says the weight lost is mostly fat mass.

• Blood sugar: it helps the pancreas release insulin when blood sugar is high, improves the body's sensitivity to insulin and lowers glucagon, a hormone that raises blood sugar.

• Stomach emptying: it slows how quickly the stomach empties, which slows the rise in blood sugar after meals. This effect becomes smaller over time.

Because it acts on GIP as well as GLP-1, it works differently from semaglutide (Wegovy), which acts on GLP-1 only.

How Long Does Mounjaro Take to Work?

Hunger falls from week 1 and fullness rises from week 3; steady levels after 4 weeks; weight loss continues over months.

The effects build in stages:

• Appetite, from the first weeks: in a study reported in the SmPC, tirzepatide decreased hunger from week 1 of treatment and increased fullness from week 3, compared with placebo.

• Steady levels after about 4 weeks: tirzepatide reaches its highest level in the blood 8 to 72 hours after each injection and has a half-life of about 5 days. Steady levels are reached after 4 weeks of weekly doses.

• Dose increases over months: treatment starts at 2.5 mg and the dose is increased in steps at least 4 weeks apart, so it takes several months to reach the higher doses. See our Mounjaro dosage guide.

• Weight, over months: in the main trial, weight loss started early and continued throughout the 72 weeks.

The first 4 weeks are on the 2.5 mg starting dose, so the full effect is not expected straight away.

What the Main Trial Found Over 72 Weeks

Average weight change at 72 weeks was -15.0% (5 mg), -19.5% (10 mg) and -20.9% (15 mg), against -3.1% on placebo.

SURMOUNT-1, published in the New England Journal of Medicine in 2022, randomly assigned 2,539 adults with obesity, or overweight with a weight-related condition, and without diabetes, to tirzepatide 5 mg, 10 mg or 15 mg or a placebo once a week for 72 weeks, alongside diet and activity advice.

Average change in body weight at 72 weeks was -15.0% on 5 mg, -19.5% on 10 mg and -20.9% on 15 mg, against -3.1% on placebo. Those averages count everyone who started, including people who stopped treatment. The SmPC reports that 89%, 96% and 96% of people on the three doses lost at least 5% of their body weight, against 28% on placebo, using the trial's estimate for people who stayed on their treatment for the full 72 weeks.

These are averages across a trial funded by Eli Lilly, which makes Mounjaro, not a prediction for one person. Results depend on the dose, how long someone stays on treatment and the diet and activity changes that go with it.

Why It Does Not Work the Same for Everyone

Responses differ; continuing is reviewed if less than 5% of weight is lost 6 months after the highest tolerated dose.

People respond differently to the same dose, and side effects can limit how high the dose goes. For weight management, the SmPC sets a review point: if someone has not lost at least 5% of their starting weight 6 months after reaching their highest tolerated dose, a decision is needed on whether to continue.

If you feel it is not working, speak to your prescriber rather than changing your dose yourself.

Side Effects That Come With How It Works

Stomach side effects such as nausea and diarrhoea, mostly while the dose is increasing.

Because Mounjaro slows stomach emptying and acts on appetite, its most common side effects are in the stomach and gut: nausea, diarrhoea, vomiting, constipation and stomach pain, mostly while the dose is increasing. Slower stomach emptying is also why you should tell the team if you are having an operation under general anaesthetic. Seek immediate medical attention for persistent, severe stomach pain, which can be a sign of pancreatitis. Because it lowers blood sugar, people with type 2 diabetes who also take insulin or a sulphonylurea have a higher risk of low blood sugar, and their prescriber may lower those doses. For the full list, see Mounjaro side effects.

Report suspected side effects to the MHRA Yellow Card scheme.

What Happens When You Stop

With a half-life of about 5 days, it leaves the body over the weeks after the last injection.

With a half-life of about 5 days, Mounjaro leaves the body over the weeks after the last injection. Talk to your prescriber before stopping. For what to expect and how people come off it, see how to come off Mounjaro.

Frequently Asked Questions