What is tirzepatide? How it works, results and side effects

What tirzepatide is

Tirzepatide is a lab-made peptide that activates two receptors your body already uses: GIP and GLP-1. It is given as a once-weekly injection under the skin. The same molecule is licensed under two brand names — Mounjaro for type 2 diabetes, and Zepbound for weight management — with different dose ranges for each use.

Adding GIP to GLP-1 is the point. People on tirzepatide generally tolerate it better and lose more weight than on a GLP-1 alone, which is why it has largely displaced older treatments in weight-clinic practice.

How it works

Tirzepatide works on the same incretin pathways that decide how hungry you feel. It dampens hunger signals in the brain, slows the rate at which food empties from your stomach, and helps the pancreas release insulin appropriately after a meal while reducing glucagon.

In practice most people describe one dominant effect: food simply stops being interesting. Snacking becomes easy to skip because the urge never arrives, and a much smaller plate feels genuinely sufficient.

What the trials showed

In the SURMOUNT programme — a family of large trials in adults with obesity or overweight who did not have type 2 diabetes — people on the highest dose lost around 20% of their body weight on average over 72 weeks, alongside diet and activity support. A clear majority lost more than 15%, and improvements in blood pressure, blood sugars and cholesterol travelled alongside the weight loss.

Treat those as population averages from a clinical trial, not a promise about you. Individual response varies a great deal — some people lose much more, some lose far less, and a small number do not respond at all. No honest clinic can tell you a number before your prescriber has reviewed you.

Side effects you should expect

The common ones are digestive and cluster around dose increases: nausea, diarrhoea, vomiting, constipation, indigestion and injection-site reactions. Most are mild to moderate and fade as the body adjusts, which is exactly why treatment starts low and climbs slowly.

• Rare but serious: pancreatitis, gallstones, and dehydration severe enough to affect the kidneys.
• Low risk of dangerous low blood sugar on its own — but the risk rises if you also take insulin or sulfonylureas, so your prescriber needs to know.
• Tell your prescriber about any history of stomach paralysis or severe gastrointestinal disease; slowed digestion is part of how the drug works and can make those conditions worse.

Who should not take it

Tirzepatide is not appropriate if you have a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2, if you are pregnant or breastfeeding, or if you have had pancreatitis. It also needs a careful conversation if you have a history of eating disorder, significant kidney or liver disease, diabetic retinopathy, or if you take other diabetes medicines.

That screening is not a formality. It is the reason these are prescription-only medicines and the reason a service that skips the questionnaire is doing something dangerous.

How dosing works with our pens

Tirzepatide is normally started at a low dose and increased gradually — commonly in steps of 2.5mg — so the body adapts and side effects stay manageable. Fixed single-dose pens force you into someone else's schedule; ours are 20mg per pen and are set to the dose your prescriber has chosen for you, so the titration matches how you are actually responding.

Cost and how to start

Tirzepatide is £100 per pen at Apex Labs. Nothing is charged at the point of order: you place the order, our prescriber approves you onto the programme, and only then does the sales team send you a secure payment link on your mobile. If you are not suitable, you are not billed.

Not sure where you stand? The eligibility check takes about three minutes and commits you to nothing.

More guides