Encyclopedia entry
How does tirzepatide work?
Oliver Mackman · Editorial director · Best Business Loans Ltd (16833937)
Last updated 2026-06-10
Tirzepatide is a once-weekly medicine that activates two gut-hormone receptors at once, GIP and GLP-1, which is why it is often called a dual agonist or twincretin. Like semaglutide it slows stomach emptying and reduces appetite through the brain, but adding GIP activity appears to strengthen the effect on appetite and how the body handles sugar and fat. In weight-management trials, average weight loss reached up to around 20 percent of body weight over roughly 72 weeks. Tirzepatide is the active ingredient in Mounjaro and is a prescription-only medicine in the UK. This page is editorial commentary, not clinical advice.
Why two hormones, not one
Most GLP-1 medicines act on a single receptor. Tirzepatide acts on two: GLP-1 and GIP, both natural incretin hormones the gut releases after eating. Combining them is the headline difference from semaglutide.
- · GLP-1 activity. Slows stomach emptying, reduces appetite via the brain, and prompts glucose-dependent insulin release.
- · Added GIP activity. A second incretin pathway that appears to add to the appetite and metabolic effect, though the exact contribution is still being studied.
- · The combined result. In head-to-head and separate trials, the dual mechanism has tended to produce larger average weight loss than single-receptor GLP-1 medicines. See GLP-1 vs GIP and how semaglutide works.
Does it work? What the trials show
In the SURMOUNT programme, tirzepatide produced average weight loss of up to around 20 percent of body weight at the highest dose over roughly 72 weeks, alongside diet and activity changes. As with any GLP-1 medicine, results vary between people and depend on staying on treatment; most of the weight tends to return after stopping, which is why aftercare and maintenance are part of the plan. For a realistic picture, see hype vs reality.
Is it safe? The UK framing
Tirzepatide is a prescription-only medicine, so a UK-registered prescriber decides whether it is appropriate and safe for you. The most common side effects are gastrointestinal, such as nausea, particularly while the dose is being increased. It carries a Black Triangle, meaning the MHRA is monitoring it closely; suspected side effects can be reported through the Yellow Card scheme. The authoritative reference for side effects and contraindications is the Patient Information Leaflet. We do not provide medical triage; whether it is safe for you is a question for your prescriber.
Frequently asked questions
Is Mounjaro the same as tirzepatide?
Yes. Mounjaro is the brand name; tirzepatide is the active ingredient inside it. They refer to the same medicine.
Is tirzepatide better than semaglutide?
In trials the dual GIP and GLP-1 mechanism has tended to produce larger average weight loss, but the right medicine depends on your clinical picture, tolerance and what your prescriber recommends. We do not recommend one medicine over another for any individual. See the licensing comparison.
What does the GIP part actually add?
GIP is a second incretin hormone. Activating it alongside GLP-1 appears to add to the appetite and metabolic effect, though researchers are still working out exactly how much it contributes.
How quickly does tirzepatide work?
Appetite changes are often felt early, but the dose is stepped up gradually over months, and the trial weight-loss figures reflect roughly 72 weeks of treatment. It is a gradual process, not an instant one.
Related: how semaglutide works · GLP-1 vs GIP · Mounjaro profile.