This one has a reasonably settled answer, so here it is. 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.

RetaRick_CA said:

The GIP arm is doing real work rather than padding the label.

Agreed on the mechanism, with the caveat that the head-to-head used semaglutide 1mg, not 2.4mg. It is still the best direct evidence available, but it is not the comparison most people think they are citing.

Admin said:

I moved from semaglutide to tirzepatide after a long stall and the first eight weeks looked like starting over — which I gather is the usual pattern…

This matches mine closely enough to be worth saying so. 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.

From the other side of the consultation, briefly.

Dose-response modeling for tirzepatide: Emax model fitting to the STEP/SURMOUNT dose-finding data shows:

Semaglutide: ED50 ≈ 0.6mg, Emax ≈ -18%, Hill coefficient ≈ 1.3
Tirzepatide: ED50 ≈ 6mg, Emax ≈ -25%, Hill coefficient ≈ 1.5

Clinical implication: most patients achieve >80% of maximal response by the mid-range dose (1.7mg sema, 10mg tirz). Going to the maximum dose provides diminishing returns — possibly not worth the additional side effect burden for some patients. Individualize dosing based on response vs tolerability.