For twenty years GIP was the incretin nobody wanted. In type 2 diabetes its insulin-releasing effect appeared blunted, and agonising it was widely assumed to be pointless or actively unhelpful for weight.
Lilly built a molecule that hits both receptors anyway. It produced the largest average weight loss of any approved medication, and the field has been revising its assumptions ever since.
The two-receptor bet
GLP-1 and GIP are the two major incretin hormones, both released from the gut in response to food, both stimulating insulin release.
The reasoning for adding GIP was not fully worked out in advance, which is unusual for a drug of this size. Several strands now look plausible: GIP appears to have its own central effects on appetite; it acts on adipose tissue in ways GLP-1 does not, influencing how fat is stored and mobilised; and there is evidence that GIP receptor activation reduces nausea, which may allow a higher effective dose to be tolerated.
That last point deserves attention, because it suggests part of the superiority may be a tolerability effect rather than a purely pharmacological one — more drug delivered, rather than better drug.
2 receptors
GLP-1 and GIP — the design decision that distinguishes tirzepatide
~5 days
Half-life, slightly shorter than semaglutide's seven
6 steps
Rungs on the tirzepatide ladder, from 2.5 mg to 15 mg
The two brands
| Brand | Licensed for | Dose ladder |
|---|---|---|
| Mounjaro | Type 2 diabetes | 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg |
| Zepbound | Weight management; obstructive sleep apnoea | Identical |
Same molecule, same ladder, different box. As with semaglutide, the practical distinction is entirely about indication and coverage.
Zepbound’s sleep apnoea approval, from SURMOUNT-OSA, has become one of the more reliable routes past a weight-loss exclusion — see Zepbound for sleep apnoea and the wider sleep picture in sleep on a GLP-1.
One detail worth knowing about the ladder: Zepbound is licensed with three maintenance doses — 5, 10 and 15 mg — with 7.5 and 12.5 formally described as intermediate steps. Stopping at 5 mg is a licensed destination, not stopping early. The full schedules are in the GLP-1 dose charts.
What the head-to-head data shows
SURPASS-2 compared tirzepatide directly with semaglutide 1.0 mg in type 2 diabetes, and tirzepatide won on both glycaemic and weight endpoints.
SURMOUNT-5 compared them for obesity, at higher doses, and again favoured tirzepatide on average weight loss.
Two caveats that matter. Average is not individual — the spread around these means is very wide, and plenty of people do better on semaglutide. And semaglutide has the longer cardiovascular outcome record, which is the relevant consideration if your reason for treatment is cardiac rather than weight. The comparison is worked through in semaglutide versus tirzepatide and the head-to-head data.
Where tirzepatide behaves differently day to day
Small practical differences that catch people out when they switch.
A shorter room-temperature window. 21 days at up to 30°C, against longer for semaglutide. This matters for travel — see travelling with your pen.
A 72-hour gap required between doses if you move your injection day, where semaglutide needs 48.
A four-day catch-up window for a missed dose, against semaglutide’s five — see missed doses.
The oral contraceptive warning. This is the significant one, and it is specific to tirzepatide: switch to a non-oral method or add a barrier for four weeks after starting and after each dose increase. With six dose steps, that is six windows. See the four-week rule and drug interactions.
Sulfur burps are more characteristically tirzepatide than semaglutide — see sulfur burps.
The second receptor was a bet placed on a hormone the field had written off. It produced the most effective weight-loss drug ever licensed, and the mechanism is still being argued about.
Getting hold of it
Lilly sells directly to self-pay patients through its LillyDirect channel, at prices substantially below list — the current landscape is in cash-pay and direct pricing and what it costs. Coverage routes, including the sleep apnoea indication, are in do you qualify and appealing a prior authorisation.
If you are moving to tirzepatide from semaglutide, the single most important thing to know is that you restart at 2.5 mg regardless of where you had reached. See switching between GLP-1s.
What comes next
Retatrutide adds a third receptor — glucagon — and is targeting effect sizes above tirzepatide. Orforglipron takes the opposite approach, abandoning the peptide structure entirely for a small molecule that can be manufactured at scale and swallowed. See the next wave and orforglipron.
Tirzepatide is currently the most effective drug in this class. On the present trajectory it will not hold that position for long, which is a good problem for everybody except the people who priced it.