The science
GLP-1 and GIP
Two gut hormones sit behind every weight-management pen on the UK market. This comparison is usually written as a contest, and the framing is wrong: they are not rivals. In the one medicine that uses both, they work together — which is the whole point of it.
Glucagon-like peptide-1
GLP-1
All three
Mounjaro, Wegovy and Saxenda all act here
- Slows how fast the stomach empties
- Signals fullness to the brain
- Prompts insulin release when blood sugar rises
- Damps down glucagon, which raises blood sugar
Glucose-dependent insulinotropic polypeptide
GIP
Mounjaro only
The part tirzepatide adds
- Also prompts insulin release after eating
- Involved in how the body handles and stores fat
- Appears to add to GLP-1’s effect rather than repeat it
- Still less well understood than GLP-1
This is not actually a versus
You cannot choose GIP instead of GLP-1. No medicine on the UK market acts on GIP alone. The real choice is between a medicine that acts on GLP-1 and one that acts on both — so the honest question is not which hormone is better, but what adding the second one buys you.
On the trial evidence, tirzepatide produces greater average weight loss than the single-hormone pens. How much of that is the GIP component specifically, rather than the molecule as a whole, is not something the trials were built to separate — and anyone who tells you otherwise is reading more into the data than it holds.
What this means when you compare pens
Side effects still overlap
A second hormone does not buy an escape from the stomach effects. Those come from GLP-1 slowing the stomach, which every pen in this class does.
GIP is the newer science
GLP-1 has decades of study behind it. GIP’s role in weight is more recent and still being worked out, which is worth knowing when you meet confident claims about it.
Mechanism is not suitability
Acting on two receptors rather than one does not make a medicine right for a given person. Your history, what else you take, and what is in stock decide far more than the receptor count.
Where this leaves you
Understanding the hormones is useful for reading claims, not for choosing a pen. “Dual agonist” is a factual description of what tirzepatide does — it is not a promise about what will happen to you, and it is often used as though it were.
If you are weighing up actual products rather than the science behind them, the medicine comparisons are the more useful place to be.
Compare the medicines instead
Sources
- Summaries of product characteristics for each product, electronic medicines compendium
- Jastreboff AM and others, SURMOUNT-1, New England Journal of Medicine, 2022
PenCompare is an independent comparison platform, not a pharmacy or a prescriber. This page is information, not medical advice, and not a recommendation to take any medicine. All three medicines named are prescription-only.