Mounjaro contains the active ingredient tirzepatide, which belongs to a class of medications called dual GIP/GLP-1 receptor agonists. It's unique among current weight loss medications because it mimics two gut hormones rather than one.
GLP-1 (glucagon-like peptide-1) is a hormone released after eating that signals to your brain that you're full, slows gastric emptying (how quickly food leaves your stomach), and helps regulate blood sugar by stimulating insulin production.
GIP (glucose-dependent insulinotropic polypeptide) is a second gut hormone that also stimulates insulin, and research suggests it may help reduce fat storage and improve how your body processes fat. The combination of both hormones acting together is thought to be why Mounjaro tends to produce slightly more weight loss than medications targeting GLP-1 alone.
By mimicking both hormones, Mounjaro reduces appetite, helps you feel fuller with smaller meals, and slows digestion — making it easier to maintain a calorie deficit without constant hunger.
Mounjaro uses a gradual dose escalation (titration) to minimise side effects. You start at a low dose and increase every 4 weeks.
| Week | Dose | Notes |
|---|---|---|
| Weeks 1–4 | 2.5mg | Starting dose — primarily for tolerability, not weight loss |
| Weeks 5–8 | 5mg | First therapeutic dose — weight loss typically begins |
| Weeks 9–12 | 7.5mg | Continued escalation |
| Weeks 13–16 | 10mg | Mid-range dose |
| Weeks 17–20 | 12.5mg | Approaching maintenance |
| Week 21+ | 15mg | Maximum maintenance dose |
You can stay on a lower dose if it's working well and you're tolerating it. Not everyone needs to reach 15mg. Your prescriber will advise based on your response and side effects.
Licensed indications in the UK: Mounjaro is licensed by the MHRA for both weight management and Type 2 diabetes in adults aged 18 and over.1
NHS eligibility: NICE approved tirzepatide for obesity treatment in December 2024 (TA1026).2 NHS access is being rolled out in phases — Cohort 1 (BMI ≥40 with 4+ comorbidities) started from June 2025, with Cohort 2 expected from June 2026.3 Only approximately 220,000 people are expected to access it on the NHS by 2028.
Private eligibility: Most private providers require a BMI of 30 or above, or 27+ with a weight-related health condition. Some may consider treatment from BMI 25 on an off-label basis after full clinical assessment.
The most common side effects are gastrointestinal and usually improve after the first few weeks or after your body adjusts to a dose increase:
Serious but rare risks include pancreatitis (~1 in 100), gallbladder problems, and severe allergic reactions. Mounjaro may also reduce the effectiveness of oral contraceptive pills and oral HRT — the MHRA recommends using additional barrier contraception for 4 weeks after starting and after each dose increase.1
View full side effects breakdown →
SURMOUNT-1: The landmark trial enrolled 2,539 adults with obesity. Participants on the 15mg dose lost an average of 22.5% of their body weight over 72 weeks, compared to 2.4% in the placebo group.4
SURMOUNT-5 (head-to-head): Published in 2025, this trial directly compared tirzepatide to semaglutide for the first time. Participants on tirzepatide lost an average of 20.2% of body weight compared to 13.7% on semaglutide over 72 weeks.5
Private prescriptions for Mounjaro typically cost £159–£329 per month, depending on the dose and provider. Higher maintenance doses (10–15mg) may cost more. Some providers charge additional consultation fees (£50–£150 for initial assessment). Following a price increase in September 2025, Mounjaro became more expensive than it was at launch, which led some patients to consider switching to Wegovy.
On the NHS, treatment is free (standard prescription charge of £9.90 applies in England) for those who qualify under the phased rollout criteria.
Read our full Mounjaro cost guide →
This page is for informational purposes only and does not constitute medical advice. Mounjaro is a prescription-only medicine. Always consult a qualified healthcare professional before starting any medication.