This is the version of the explanation I wish somebody had given me, written down before I forget what confused me. It is about tirzepatide, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
SURMOUNT-1 landed around 20.9% mean weight loss at 15mg over 72 weeks, against roughly 15% for semaglutide 2.4mg in STEP 1. Different trials, different populations, so the comparison is indicative rather than decisive — but SURPASS-2 was a genuine head-to-head and it pointed the same way.
The condition it depends on
The ladder is longer and that is not a neutral detail — six dose steps means six opportunities to stall on the way up, and plenty of people never reach the dose the headline number came from.
The practical version
Practical numbers: half-life about 5 days, steady state 3 to 4 weeks, ladder 2.5 / 5 / 7.5 / 10 / 12.5 / 15mg, and the maintenance doses with published data behind them are 5, 10 and 15mg.
What I am not sure about
So the question, as narrowly as I can put it: how much of the tirzepatide advantage is the GIP component and how much is simply that the dose ladder goes higher in receptor-occupancy terms. I have searched first, so if this is covered somewhere point me at it and I will read it.
emma_london said:SURMOUNT-1 landed around 20.9% mean weight loss at 15mg over 72 weeks, against roughly 15% for semaglutide 2.4mg in STEP 1.
That is correct as far as it goes, and here is where it stops going. The GIP arm is doing real work rather than padding the label. GIP receptor agonism appears to improve adipose insulin sensitivity and lipid handling, and — counter-intuitively — GIP signalling in the CNS reduces nausea rather than adding to it, which is why tolerability at high total agonism is better than the GLP-1-only comparison would predict. SURPASS-2 is the cleanest head-to-head: tirzepatide beat semaglutide 1mg at every dose tier.
emma_london said:SURMOUNT-1 landed around 20.9% mean weight loss at 15mg over 72 weeks, against roughly 15% for semaglutide 2.4mg in STEP 1.
Filing a mild objection. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit. The "tirzepatide is simply better" summary irritates me. It is better on mean weight loss, and the cardiovascular outcome evidence is far thinner than semaglutide's. If the reason for treating is cardiovascular risk rather than weight, the evidence base points the other way.
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Shop Reference StandardsAnswering the narrow version, because the broad one does not have a single answer. Steady state is faster than people expect: half-life about five days, so you are at plateau in roughly three to four weeks rather than five. That matters when you are deciding whether a dose step has finished expressing itself.
NeuroNate said:The GIP arm is doing real work rather than padding the label.
Agreed, with the qualification that SELECT enrolled a secondary-prevention population. Extrapolating a 20% relative reduction to a healthy 35-year-old with a BMI of 31 is not what that trial showed.