What is tirzepatide? The dual GIP/GLP-1 drug, explained

What tirzepatide actually does

Your gut releases hormones called incretins after you eat. Two of them — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — tell the pancreas to release insulin, slow how fast the stomach empties, and dial down appetite. Tirzepatide is a single molecule engineered to press both buttons at once, which is why StatPearls and the original discovery paper both label it a "dual GIP and GLP-1 receptor agonist." Think of it as a light switch wired to two circuits instead of one.

That dual action is the headline difference from semaglutide. In the SURMOUNT-1 obesity trial, adults on the 15 mg dose lost roughly 21% of body weight over 72 weeks, versus about 3% on placebo. The point of this guide is not to recommend a dose — that is a prescriber's job — but to explain what the drug is and where the dose math fits, so the number on your syringe matches the number on your prescription.

Mounjaro vs Zepbound vs semaglutide

Same active ingredient, two brand names: Mounjaro is tirzepatide approved for type 2 diabetes, Zepbound is tirzepatide approved for chronic weight management (and obstructive sleep apnoea). Both run the same 2.5 mg-to-15 mg ladder. The table compares the three injectables people most often confuse.

The milligram numbers are not interchangeable across rows. 2.5 mg of tirzepatide is a different drug at a different potency from 2.4 mg of semaglutide — never copy a dose across brands. Retatrutide is still in trials, so its human dosing is not established; treat any number for it as experimental. For the GLP-1-only sibling, see our plain-English semaglutide explainer.

How the dose ladder works

Tirzepatide is titrated, meaning you climb slowly. Both FDA labels say to start at 2.5 mg once weekly for 4 weeks — an initiation dose that is explicitly not a maintenance dose — then increase by 2.5 mg increments after at least 4 weeks on each step. The slow climb gives the gut time to adapt and keeps nausea manageable. The staircase below shows the standard escalation.

Not everyone reaches 15 mg. Many people settle at 5, 10 or 12.5 mg if that controls symptoms or weight, because the maintenance doses for weight management are 5, 10 or 15 mg. The ladder is about tolerance, not a race to the top. For the calendar side of this, see the full tirzepatide dosing guide and what a titration schedule is.

How this is calculated

A dose is the amount of drug (mg). The volume is how much liquid carries it (mL). Concentration links them: mg per mL. The brand pens are pre-filled, so most patients never do this math — but if you draw from a vial, the formula is simply volume (mL) = dose (mg) ÷ concentration (mg/mL), then units = mL × 100 on a U-100 syringe. Below are worked examples across the common strengths and vial sizes; check any of them in the tirzepatide calculator.

Common beginner mistakes

The biggest error is copying someone else's unit count. "It's 25 units" only holds if your vial concentration matches theirs — 25 units of a 10 mg/mL vial is 2.5 mg, but 25 units of a 20 mg/mL vial is 5 mg. The mark is the same; the dose is double. A second trap is treating semaglutide and tirzepatide numbers as the same; they are different molecules on different ladders. A third is forgetting dead space — the drop left in the needle hub — which matters most when the draw is tiny.

Storage and technique can make correct math unsafe. Use a fresh sterile needle and syringe, follow the product's refrigeration instructions, and never use a vial or pen that is cloudy, discolored, expired or damaged.

So, what is tirzepatide?

Tirzepatide is a once-weekly injectable that activates both the GIP and GLP-1 receptors simultaneously, making it a dual agonist that reduces appetite and blood sugar more strongly than single-pathway drugs. It is approved as Mounjaro (type 2 diabetes) and Zepbound (weight management), and it climbs a 2.5 mg-to-15 mg titration ladder in 2.5 mg steps no faster than every 4 weeks. To convert your prescribed dose into syringe units, use the tirzepatide dosage calculator.

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