Tirzepatide: What It Is and How It Works
What tirzepatide is, how it activates the GIP and GLP-1 receptors at the same time, and what the SURPASS and SURMOUNT trials actually found.
Tirzepatide is a synthetic 39-amino-acid peptide that activates two receptors at once: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). That dual action is what sets it apart from earlier compounds in the class.
The FDA approved it in May 2022 as Mounjaro for type 2 diabetes, and in November 2023 as Zepbound for the treatment of obesity.
How it works: two receptors instead of one
Incretins are hormones the gut releases after a meal. They signal the pancreas to produce insulin and tell the brain the body has been fed. Tirzepatide mimics two of them simultaneously.
The GLP-1 receptor
Activating the GLP-1 receptor has three main effects: it stimulates insulin secretion only when blood glucose is elevated, it suppresses glucagon release, and it slows gastric emptying. That last one is the reason for the sense of fullness many people describe.
The GIP receptor
GIP is the other major incretin hormone. Its role stayed unclear for a long time — in people with type 2 diabetes, the response to GIP is blunted. Research indicates that activating GIP alongside GLP-1 produces a better metabolic result than GLP-1 alone, likely through effects on adipose tissue and an additional contribution to appetite regulation.
What the trials found
SURPASS — type 2 diabetes
The SURPASS programme studied tirzepatide in type 2 diabetes. SURPASS-2 compared it head-to-head against semaglutide 1 mg over 40 weeks. All three tirzepatide doses studied achieved a greater reduction in HbA1c than semaglutide, along with greater reductions in body weight.
SURMOUNT — obesity
SURMOUNT-1 followed 2,539 adults without diabetes for 72 weeks. Mean change in body weight reached approximately 20.9% at the highest dose studied, versus about 3.1% on placebo — among the largest figures reported for pharmacological treatment of obesity.
The numbers above are outcomes from controlled clinical trials run under defined protocols with medical supervision. They are not a prediction of any individual result and are not a basis for self-medication.
Pharmacokinetics: why once weekly
Native GLP-1 is broken down within minutes by the enzyme DPP-4. To make dosing practical, tirzepatide was modified with a fatty acid side chain that binds reversibly to albumin in the blood. Albumin acts as a reservoir, releasing the molecule gradually and extending the half-life to roughly 5 days.
One consequence is that steady plasma levels are only reached after several weeks — which is why clinical protocols escalate gradually rather than starting at a full dose.
Side effects
The profile is predominantly gastrointestinal: nausea, diarrhoea, vomiting, constipation, reduced appetite. In the trials these were most pronounced during dose escalation and eased once the dose stabilised. Discontinuation due to adverse effects occurred in a small proportion of participants.
Full information on contraindications, interactions and warnings is in the official prescribing information listed under sources.
Why tracking matters
With a weekly schedule running over months, memory is an unreliable instrument. Three things are worth recording consistently:
- Exactly when each dose was administered — so the interval is precise rather than approximate.
- What the dose was — particularly during periods of change.
- Weight and subjective notes — so you see a trend rather than isolated numbers.
That is exactly what PeptCycle does: a calendar, reminders, injection history and a weight chart — stored locally on the device, with no account and no server.
Frequently asked questions
Is tirzepatide the same as semaglutide?
No. Semaglutide acts on the GLP-1 receptor only, while tirzepatide activates two receptors — GIP and GLP-1. Both belong to the same family of incretin analogues, but they have different activity profiles and different trial outcomes.
Why is it given once a week?
The molecule carries a fatty acid side chain that binds reversibly to albumin in the blood. This slows its clearance and extends the half-life to roughly 5 days, which makes weekly administration practical.
What is the most common side effect?
Gastrointestinal complaints are the most frequently reported in clinical trials — nausea, diarrhoea, vomiting and constipation. They are usually mild to moderate and tend to decrease over time.
Does PeptCycle recommend tirzepatide doses?
No. PeptCycle is a record-keeping and tracking tool. Every value is entered by the user, and the calculator only converts millilitres into syringe units. The app contains no recommended doses anywhere.