Weight loss injections: How do drugs like Wegovy and Mounjaro work?

There are two drugs on the market – semaglutide, marketed under the brand name Wegovy, and tirzepatide, sold as Mounjaro. Semaglutide is also used in the diabetes treatment Ozempic.

Both Wegovy and Mounjaro are given as weekly injections via pre-filled pens that can be self-administered into the upper arm, thigh or stomach.

They work as an appetite suppressant by mimicking a hormone called glucagon-like peptide-1 (GLP-1).

This intestinal hormone is released after eating, and typically makes people feel fuller.

Mounjaro also affects another hormone, glucose-dependent insulinotropic polypeptide (GIP), which affects metabolism and helps regulate energy balance.

Patients typically start on a low dose which is gradually increased until they reach a higher maintenance dose.

People taking these drugs typically start to lose weight within a few weeks.

Clinical trials suggest that – when combined with changes to their diet, exercise and behavioural support – Wegovy users could lose more than 10% of their body weight after a year on the treatment.

Other trials suggest Mounjaro users might lose even more. But it is difficult to compare the two treatments and crucially, user of either can put weight back on once they stop treatment.