Tirzepatide (Mounjaro) in the UK

What tirzepatide is

Tirzepatide is a synthetic peptide that activates both the GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors. The dual mechanism is the headline difference from semaglutide, which acts on GLP-1 alone.

In the UK it is marketed as Mounjaro. Weekly subcutaneous injection; dose-titration over several weeks to manage side effects.

How tirzepatide compares with semaglutide

Head-to-head trial data (SURPASS, SURMOUNT) suggest tirzepatide produces somewhat greater mean weight loss than semaglutide at maximum tolerated dose — figures in the high teens to low twenties (%) at 72 weeks. But individual response varies, and the practical choice between them depends on:

• Prescriber availability for the specific indication.
• NHS access — NICE positions differ between the two for weight management.
• Side-effect tolerance — both share the GLP-1 GI side-effect profile; tirzepatide may have a slightly different feel.
• Cost and supply — private pricing varies; supply has been tight.

The choice should be made by the prescriber with you, not by benchmarking trial averages against your goals.

UK access — the lawful routes

1. NHS — access depends on NICE technology appraisal position and local commissioning. Check the current NICE position with your GP; eligibility criteria are specific and have changed over time.
2. Private prescription from a UK-registered prescriber, dispensed by a UK-regulated pharmacy. As with semaglutide, real clinical consultations are the standard; a 60-second tick-box form is not.

Anything else — vials labelled "tirzepatide" sold for "research", or "Mounjaro" sold without prescription on social media — sits outside the regulated supply chain.

What tirzepatide actually does in practice

• Slows gastric emptying.
• Reduces appetite via central pathways.
• Increases glucose-dependent insulin secretion; suppresses inappropriate glucagon.
• Mean weight loss in SURMOUNT-1 (tirzepatide 15mg) was in the low-to-mid 20s (%) at 72 weeks; individual variation is large.

The supply-chain question — be alert

Demand for tirzepatide has been intense and supply has been tight. That has driven a significant black market in "generic tirzepatide" vials and counterfeit Mounjaro pens. The MHRA has warned the public. Practical safety implications:

• There is no licensed generic of tirzepatide at the time of writing.
• "Compounded tirzepatide" offers in the UK are almost always unregulated grey-market product.
• Counterfeit Mounjaro pens have been seized that contained the wrong active ingredient (including insulin), causing severe hypoglycaemia.

If a private clinic is sourcing tirzepatide from anywhere other than a UK-regulated pharmacy supply chain, that is the time to ask detailed questions. See fake weight-loss pens.