Is Mounjaro a Hormone — or Does It Work With Your Hormones?

What exactly are GIP and GLP-1, and why does Mounjaro target both?

GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are incretin hormones released from the gut wall in response to food. Their jobs overlap but are not identical. GLP-1 slows the rate at which the stomach empties, blunts hunger signals reaching the brain, and prompts the pancreas to release insulin in proportion to blood glucose. GIP works alongside it, amplifying the insulin response and playing a role in fat tissue metabolism.

In people with obesity, these signals can become less effective, a phenomenon sometimes described as incretin resistance. Tirzepatide is engineered to activate both receptors simultaneously, which is why you will sometimes see it described as a dual GIP and GLP-1 receptor agonist. It is not extracted from the body, nor is it a replacement for a deficient hormone the way insulin is. It is a molecule designed to bind to those receptors more reliably and for longer than the natural hormones do.

The NHS tirzepatide medicines page describes this mechanism plainly and is a useful first reference if you want to understand how the medicine interacts with your body before discussing it with a prescriber.

How does this differ from semaglutide, is tirzepatide a hormone too?

The same question (is tirzepatide a hormone) comes up because medicines like Wegovy (semaglutide) are often grouped with Mounjaro under the GLP-1 label. Semaglutide is a single-pathway medicine: it targets the GLP-1 receptor only. Tirzepatide adds the GIP receptor on top. Neither is a hormone; both are synthetic receptor agonists.

The practical difference matters clinically. In the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight reduction over 72 weeks than semaglutide 2.4mg in adults with obesity and no diabetes. NICE's appraisal of tirzepatide (TA1026) references this evidence when setting out the recommendation. Whether the dual-receptor action is the reason for the larger effect, or whether other pharmacological factors contribute, remains an active area of research.

If you are weighing up the two medicines and want a fuller picture of how their mechanisms compare day to day, the Mounjaro and hormones overview goes into more depth on what each pathway does in practice.

Does activating these hormone receptors affect other hormones in your body?

A fair question, and one our prescribers hear regularly from people who want to understand the full picture before starting treatment. The honest answer is: GIP and GLP-1 receptors are not only in the gut. They are present in the brain, the pancreas, the cardiovascular system and elsewhere, which is why tirzepatide has effects that extend beyond appetite alone, including cardiovascular data now being studied in longer-term trials.

For women specifically, there is a well-documented interaction with oral contraceptive pills. Because Mounjaro slows gastric emptying, the absorption of oral medicines taken around the same time can be affected. How tirzepatide affects female hormones covers the contraception guidance in detail, but the short version is this: if you take the pill, your prescriber will advise adding a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase. NHS England makes the same recommendation in its weight-management-injections guidance. This is not evidence that tirzepatide disrupts your sex hormones directly, it is a precaution around absorption timing.

There are also questions about thyroid hormones, HRT and whether GLP-1 receptor activation influences other endocrine pathways. These are legitimate areas of ongoing research, and the evidence on tirzepatide and hormone balance covers what is currently known and what remains uncertain.

So is Mounjaro safe to take alongside other hormone-based treatments?

This is ultimately a clinical question, not one answered correctly by a general page. What the evidence and current UK guidance say is that Mounjaro is not itself a hormone, does not replace or suppress your body's own hormone production in the way that, for example, thyroid medication or HRT does, and is not contraindicated with most hormone-based treatments as a class.

That said, the gastric-emptying effect is real and affects how other oral medicines behave. NHS England guidance specifically mentions considering transdermal HRT (patches or gels rather than tablets) for women on tirzepatide, precisely because oral absorption may be less predictable. Anyone on thyroid replacement therapy, diabetes medication or other endocrine treatments should make sure their prescriber knows the full picture.

If you are considering treatment and want to understand how it sits alongside your existing medication, the Mounjaro and hormone interactions page sets out the current clinical position. You can also check your eligibility through a free consultation, a GPhC-registered prescriber reads every submission the same day and will flag any interactions before approving anything, and if you would like to review the available dose options before that conversation, the Mounjaro UL page sets out the full details.