Writing this once so I can stop repeating it across threads. It is about the pharmacology, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
The mechanism is more central than most summaries suggest. Receptor agonism in the arcuate nucleus activates POMC neurons and inhibits AgRP/NPY signalling, and the downstream MC4R pathway is the same one disrupted in monogenic obesity — convergent genetic evidence that the target is the right one. Peripherally there is glucose-dependent insulin secretion, glucagon suppression and delayed gastric emptying, but the gastric component largely adapts over months while the central effect persists, which is why the durable effect is appetite rather than fullness.
The condition it depends on
The adaptation point cuts both ways: tachyphylaxis to gastric emptying is why tolerability improves, and it is also why people who were relying on physical fullness feel the effect fade while the appetite effect is still working.
The practical version
Numbers worth memorising for this class: Tmax one to three days for the weekly peptides, terminal half-life about a week for semaglutide and about five days for tirzepatide, steady state at four to five half-lives, subcutaneous bioavailability high enough that site choice is irrelevant.
What I am not sure about
The question I want answered is which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working. I would rather have one careful answer than five confident ones.
SarahChen_PharmD said:The mechanism is more central than most summaries suggest.
Agreed, though "tolerable" needs defining. A dose you tolerate by eating almost nothing is not tolerated, it is being paid for somewhere else — usually in lean mass, sometimes in adherence three months later.
SarahChen_PharmD said:The mechanism is more central than most summaries suggest.
Pushing back on SarahChen_PharmD here. The pharmacokinetics explain nearly every practical question asked here. Albumin binding above 99% slows clearance enough to make weekly dosing possible; a terminal half-life near a week means four to five weeks to steady state and therefore a four-week titration interval; subcutaneous bioavailability around 89% means injection site barely matters. Those three facts answer most timing questions before they are asked.
If somebody has the primary source to hand I would rather cite it than paraphrase it.
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View ResultsShort answer first, then the reasoning. The mechanism that matters here is not stomach emptying, it is central. GLP-1 receptor agonism in the arcuate nucleus stimulates POMC neurons and suppresses AgRP/NPY signalling, which is why the effect is appetite and food salience rather than physical fullness. Delayed gastric emptying largely tachyphylaxes over the first months; the appetite effect does not.
Dr.KarenChen said:Agreed, though "tolerable" needs defining.
All true, with one condition: that curve is for people who reached the dose on schedule. Anyone who slowed the ladder for tolerability is on a different, flatter curve, and comparing yourself with the published mean will make you feel like a non-responder when you are not.