Tirzepatide is the most effective weight loss medication currently approved in the UK. In the SURMOUNT-1 trial, published in the New England Journal of Medicine in 2022, adults with obesity but without diabetes lost an average of 20.9 percent of their body weight at the 15mg dose over 72 weeks. The SURMOUNT-2 trial in adults with type 2 diabetes found average weight reductions of 14.7 percent at the same dose. These figures exceed anything achieved by semaglutide or liraglutide in comparable trials and have established tirzepatide as the benchmark against which newer drugs are measured.
The Dual Mechanism
Tirzepatide is classified as a dual GIP and GLP-1 receptor agonist. GIP, or glucose-dependent insulinotropic polypeptide, is a second gut hormone that works synergistically with GLP-1 to enhance insulin secretion, reduce glucagon, and modulate fat storage. The combination of these two mechanisms appears to produce greater appetite suppression and metabolic improvement than GLP-1 agonism alone, accounting for tirzepatide's superior efficacy. It is sold under the brand name Mounjaro in the UK for both diabetes management and weight loss.
Gastrointestinal Side Effects
In the SURMOUNT-1 trial, nausea affected approximately 31 percent of participants at the 15mg dose, vomiting 12 percent, diarrhoea 23 percent, and constipation 11 percent. These rates are somewhat lower than those reported for semaglutide in the STEP trials, despite tirzepatide's greater efficacy, which may relate to differences in the dose titration schedules and trial designs. As with semaglutide, GI side effects are most pronounced during dose escalation and tend to attenuate with continued treatment. Injection site reactions are common and improve with site rotation.
Muscle Loss: The Largest Absolute Risk of the Drug Class
Because tirzepatide produces greater total weight loss than semaglutide or liraglutide, it also produces the greatest absolute loss of lean mass of the three drugs when used without specific muscle-preservation strategies. If approximately 30 to 40 percent of weight lost represents lean mass, a participant losing twenty-two kilograms on tirzepatide may lose seven to nine kilograms of lean tissue. This is the equivalent of approximately a decade of natural age-related muscle loss imposed over eighteen months. For adults in their fifties and sixties who are already contending with sarcopenic decline, this is the most significant clinical consideration associated with tirzepatide use.
A 2024 analysis of the SURMOUNT-4 trial, which examined tirzepatide maintenance treatment, confirmed that lean mass was substantially reduced during the initial weight loss phase. Participants who had embedded resistance training into their routine before and during treatment showed attenuated lean mass loss, consistent with findings across the GLP-1 drug class. The volume of evidence now supports treating resistance training not as an optional complement to GLP-1 therapy but as a clinical requirement for adults over 35.
Other Safety Considerations
Tirzepatide carries the same class-wide boxed warning for thyroid C-cell tumours and is contraindicated in the same populations as semaglutide and liraglutide. Pancreatitis risk is similar to the rest of the class. Gallbladder disease, including cholelithiasis and acute cholecystitis, is increased by the same mechanism of rapid weight loss. Hypoglycaemia risk is higher in patients concurrently taking sulphonylureas or insulin, and dose adjustments of those medications are typically required.
- Average weight loss 20.9% at 15mg dose over 72 weeks: the highest of any approved weight loss drug
- Dual GIP and GLP-1 agonist: distinct mechanism from semaglutide and liraglutide
- Nausea (31%), diarrhoea (23%), vomiting (12%), constipation (11%) at 15mg dose
- Absolute lean mass loss is the highest of the three drugs due to greater total weight reduction
- Participants losing 22kg may lose 7 to 9kg of lean mass without resistance training
- Thyroid, pancreatitis, and gallbladder warnings apply as with the rest of the drug class
- Sold as Mounjaro in the UK; weekly subcutaneous injection; dose range 2.5mg to 15mg