Tirzepatide

Also called Mounjaro, Zepbound.

Dr Chad OkayFounder, Suna Health

4 min read. Updated .

Tirzepatide is a synthetic peptide that activates two gut-hormone receptors at once: GLP-1 and GIP. It is made by Eli Lilly and sold in the UK as Mounjaro for both type 2 diabetes and weight management. The dual mechanism produces stronger weight-loss results than semaglutide in clinical trials, with side effects of similar character.

What makes it different

Most GLP-1 drugs target just the GLP-1 receptor. Tirzepatide is the first approved drug to target both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). GIP is another incretin hormone, also released by the gut after eating. The combination produces stronger insulin release and more weight loss than either signal alone, at least in current trials. Drugs that target three receptors at once (like retatrutide, in late-stage trials) are next.

How weight loss compares to semaglutide

In the SURMOUNT-1 trial of tirzepatide for obesity, participants on the highest dose (15 mg weekly) lost an average of 20.9% of body weight over 72 weeks (the efficacy estimand, for people who stayed on the drug, was 22.5%). The same trial design with semaglutide (STEP-1) showed 14.9%. In the first head-to-head trial, SURMOUNT-5, tirzepatide produced 20.2% weight loss versus 13.7% for semaglutide. Real-world results vary, and the SURPASS trials in diabetes showed slightly smaller differences. NICE approved tirzepatide for obesity in the UK in 2024 (TA1026), with eligibility limited to a body mass index (BMI) of 35 or above with weight-related conditions, plus structured behavioural support.

Doses and titration

  • Starting dose: 2.5 mg once weekly, for 4 weeks.
  • Step 2: 5 mg weekly. Many people stay here.
  • Higher doses: 7.5 mg, 10 mg, 12.5 mg, 15 mg. Titrated up at 4-week intervals if tolerated and required.
  • Each dose step typically triggers a 1 to 2 week recurrence of side effects before the gut adapts.

Side effects

The side-effect profile is similar to semaglutide. Nausea is the most common, peaking around the high 20s percent at the higher doses (Zepbound label: about 25% / 29% / 28% across 5/10/15 mg). Constipation does NOT rise linearly with dose, the Zepbound label reports about 17% on 5 mg, 14% on 10 mg, and 11% on 15 mg. Sulphur burps are reported more often by tirzepatide users than semaglutide users in real-world reporting. Most symptoms peak in weeks 2 to 4 of each dose escalation and ease as the gut adapts.

How it is taken in the UK

Mounjaro is prescription-only. It is available privately through online and high-street pharmacies, and via the NHS for diabetes. NHS access for obesity is being phased in from 2025, starting with people with BMI 40 or above with four weight-related comorbidities. Private prices in the UK ranged from around £120 to £340 per month in 2026, depending on dose and pharmacy.

If something feels wrong

This entry is reference information, not medical guidance. Your doctor and pharmacist know you and your medicine; if anything about how you feel worries you, speak to them promptly, and if you ever feel seriously unwell, treat it as an emergency. The medicine's own patient information leaflet carries the important safety information and is worth reading in full.

Common questions

Is Mounjaro stronger than Ozempic?
On weight loss, yes. In the first head-to-head trial (SURMOUNT-5), tirzepatide produced 20.2% average weight loss compared with 13.7% for semaglutide. Side effects are similar in frequency and character.
What is the difference between Mounjaro and Zepbound?
Both contain tirzepatide and are made by Eli Lilly. Mounjaro is the brand used in the UK and EU for both diabetes and obesity. Zepbound is the brand used in the United States for obesity only. The drug is identical.
How long do you stay on tirzepatide?
Clinical guidance treats GLP-1 receptor agonists as long-term therapy in most cases, similar to medications for blood pressure or cholesterol. NICE TA1026 explicitly does NOT cap tirzepatide at two years; treatment can continue long-term. The review trigger is at six months on the highest tolerated dose: continue if at least 5% of starting weight has been lost, otherwise stop. Stopping leads to substantial weight regain in most people. In the SURMOUNT-4 trial, people switched to placebo regained on average about 14% of body weight over the following year, while those who stayed on treatment lost a further 5.5%.
Why do tirzepatide users get sulphur burps?
Slowed gastric emptying means food and stomach contents sit longer than normal. Gut bacteria have more time to ferment sulphur-containing amino acids from food, producing hydrogen sulfide gas, which causes the characteristic egg-like smell. The simplest fix is smaller meals, slower eating, and giving the gut time to clear before sleeping.

Sources

Written by Dr Chad Okay. Contributor: Dr Parisah Hussain, MBChB, BSc (Hons), DRCOG, MRCGP. Last updated 17 September 2026. From Digests.

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