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ForumsPublic SquareComparative pharmacokinetics: semaglutide vs tirzepatide vs retatrutide — anyone have experience?

Comparative pharmacokinetics: semaglutide vs tirzepatide vs retatrutide — anyone have experience?

Dr.AddMedPHL Mon, Dec 8, 2025 at 2:21 PM 5 replies 965 viewsPage 1 of 1
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Dr.AddMedPHL
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Dec 8, 2025 at 2:21 PM#1

Reading about receptor desensitisation and trying to work out which of the effects adapt over time and which do not, because people clearly experience both.

What would genuinely help is knowing which effects tachyphylax and which persist, because the answer explains why tolerability improves while the appetite effect keeps working.

Tell me what I have not thought of.

26 21FitDadDave, RunnerRach, TrialNerd_Beth and 23 others
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PurityPaulOR
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Dec 8, 2025 at 2:46 PM#2

Short 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.

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Dr.Martinez
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Dec 8, 2025 at 3:11 PM#3
PurityPaulOR said:
The mechanism that matters here is not stomach emptying, it is central.

That is correct as far as it goes, and here is where it stops going. 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.

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kevin_tulsa
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Dec 8, 2025 at 3:36 PM#4
Dr.AddMedPHL said:
Reading about receptor desensitisation and trying to work out which of the effects adapt over time and which do not, because people clearly experience…

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.

23 18quinn_sf, NurseLeah_Nash, gary_naperville and 20 others
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rachel_ABQ
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Dec 8, 2025 at 5:52 PM#5

From the other side of the consultation, briefly.

Amycretin (AMY/GLP-1 dual agonist) emerging data relevant to the pharmacology: Phase 1 showed -13.1% body weight at only 12 weeks, the fastest trajectory ever seen for an anti-obesity agent[1].

Amylin receptor agonism enhances satiety signaling through the area postrema and reduces glucagon secretion. Combined with GLP-1R agonism, this dual mechanism may produce even greater efficacy than current agents.

Early-stage data — interpret with caution. But the trajectory is extraordinary.

References:
[1] Novo Nordisk investor presentation, September 2023.
Last edited: Dec 8, 2025 at 10:52 PM
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