"How does Mounjaro actually work?" is the question I hear most often from patients considering weight loss treatment. It's a fair question, and it deserves a proper clinical answer, not a marketing summary. In this article I'll explain exactly what tirzepatide does in your body, what the trial data shows, and what you can realistically expect if you're prescribed it.
What is Mounjaro?
Mounjaro is the brand name for tirzepatide, a prescription-only injectable medication manufactured by Eli Lilly. It was first licensed in the UK for type 2 diabetes in 2022, and then received MHRA approval for weight management in November 2023. It is the first dual-action GLP-1/GIP receptor agonist to be licensed for obesity in the UK.
Mounjaro is administered as a once-weekly subcutaneous injection using a prefilled KwikPen. You inject it into your stomach or thigh, rotating the site each week. The back of your upper arm can also be used if someone else gives the injection.
The two-hormone mechanism: why Mounjaro is different
GLP-1 and GIP are gut hormones that your body releases naturally after eating. Older weight loss injections like Wegovy and Saxenda mimic GLP-1 only. Mounjaro is the first medication to mimic both GLP-1 and GIP simultaneously. Scientists originally doubted whether dual action would improve outcomes compared to GLP-1 alone, but the clinical data proved otherwise convincingly.
GLP-1 activation
- Signals the hypothalamus (the brain's appetite control centre) that you're full
- Slows gastric emptying: food moves more slowly from your stomach
- Stimulates the pancreas to release insulin when blood glucose rises
- Suppresses glucagon, the hormone that raises blood sugar between meals
- Reduces cravings and "food noise" via brain reward pathways
GIP activation
- Promotes fat metabolism and reduces fat storage in fat cells
- Enhances the appetite-suppressing effects of GLP-1
- Improves insulin sensitivity in peripheral tissues
- Acts on the central nervous system to regulate energy balance
How tirzepatide works in your body
GLP-1 Receptor
Appetite Control
Signals the hypothalamus to reduce hunger, slows gastric emptying, and quietens food cravings.
↓ Appetite
GIP Receptor
Fat Metabolism
Promotes fat breakdown in fat cells, improves insulin sensitivity, and enhances GLP-1's appetite-suppressing effects.
↓ Fat storage
Dual Action
Superior Results
Tirzepatide is the only medication to activate both receptors simultaneously. In SURMOUNT-5 (New England Journal of Medicine, 2025), a 72-week trial comparing the two medicines directly in 751 adults with obesity but not diabetes, people on the highest dose they could tolerate of tirzepatide lost an average of 20.2% of their body weight, compared with 13.7% on semaglutide. Individual results vary.
20.9% vs 3.1%: tirzepatide 15mg vs placebo over 72 weeks (SURMOUNT-1).
Sources: Jastreboff AM et al. N Engl J Med 2022 (SURMOUNT-1); Aronne LJ et al. N Engl J Med 2025 (SURMOUNT-5)
In the SURMOUNT-1 trial (New England Journal of Medicine, 2022), adults without diabetes who took tirzepatide 15mg alongside diet and activity advice lost an average of 20.9% of their body weight over 72 weeks, compared with 3.1% on placebo, counting everyone who started treatment. Among people who stayed on treatment, the average was 22.5%. On 15mg, 91% lost at least 5% of their weight, compared with 35% on placebo. Individual results vary.
Source: Jastreboff AM et al. N Engl J Med 2022;387:205 to 216.
How quickly does Mounjaro work?
Most patients notice reduced appetite within the first week of treatment. Meaningful, measurable weight loss typically begins at 4 to 8 weeks. In the SURMOUNT-1 trial (New England Journal of Medicine, 2022), adults without diabetes who took tirzepatide alongside diet and activity advice lost an average of 15.0% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% on placebo, counting everyone who started treatment. Among people who stayed on treatment, the averages were 16.0%, 21.4% and 22.5%. Individual results vary.
Dosing and titration
Treatment starts at 2.5mg once weekly. The dose increases by 2.5mg every four weeks according to tolerability, up to a maximum of 15mg. The recommended maintenance doses in the UK product information are 5mg, 10mg and 15mg, and you do not need to reach 15mg. In the SURMOUNT-1 trial (New England Journal of Medicine, 2022), adults without diabetes lost an average of 15.0% of their body weight on 5mg and 19.5% on 10mg over 72 weeks, compared with 3.1% on placebo, counting everyone who started treatment. Individual results vary.
The low starting dose is deliberate: it minimises gastrointestinal side effects while your body adjusts. Rushing the titration schedule is the leading cause of difficult side effects, which is why your pharmacist will review your progress before each dose increase.
Who is Mounjaro suitable for?
Mounjaro is suitable for adults with a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related condition such as hypertension, type 2 diabetes, dyslipidaemia, or obstructive sleep apnoea.
When we will not supply Mounjaro
The UK product information lists one contraindication: an allergy to tirzepatide or any of the other ingredients. It also says Mounjaro should not be used during pregnancy. We do not supply Mounjaro if, for example, you are pregnant, planning a pregnancy or breastfeeding, have had pancreatitis, have type 1 diabetes, or if you or a blood relative have had medullary thyroid cancer (MTC) or multiple endocrine neoplasia type 2 (MEN2).
An important note for patients taking oral contraceptives: you should use an additional barrier method of contraception for four weeks after starting Mounjaro and for four weeks after each dose increase. Tirzepatide slows gastric emptying and may reduce the absorption of oral medications, including the contraceptive pill.
Side effects
The most common side effects are gastrointestinal and occur mainly during dose increases. Nausea is very common. In the weight-management trials in the UK product information (SURMOUNT-1 and SURMOUNT-2), it affected 24.6% to 29.0% of people on tirzepatide, depending on the dose, compared with 8.5% on placebo, and was most common while the dose was being increased. Individual results vary. Diarrhoea, vomiting, and constipation are also very common. Most of these symptoms improve as your body adjusts, typically over 4 to 8 weeks at each dose level.
Serious side effects include pancreatitis, which is uncommon (up to 1 in 100 people). Gallstones are common in people taking Mounjaro for weight loss (up to 1 in 10 people). Stop your treatment and seek urgent medical help if you have severe, persistent stomach pain, especially if it spreads to your back, or sudden pain in the upper right of your tummy, fever, or yellowing of the skin or eyes, which can be signs of gallbladder problems.