InsuranceTom said:

I went looking for why the dosing schedule is what it is and found that almost every practical question on this board has a pharmacokinetic answer…

Receptor pharmacology relevant to the pharmacology: semaglutide is a GLP-1R agonist with a C-18 fatty acid chain that enables albumin binding (>99%), creating a depot effect with a ~168-hour half-life enabling weekly dosing[1].

Tirzepatide is a dual GIP/GLP-1R agonist with higher GIP affinity (5:1 GIP:GLP-1 potency ratio). The GIP component may enhance beta-cell function and adipocyte lipid metabolism beyond what GLP-1 alone achieves.

For the pharmacology, the pharmacology explains the clinical differences between these agents.

CarlaRPh_TPA said:

Receptor pharmacology relevant to the pharmacology: semaglutide is a GLP-1R agonist with a C-18 fatty acid chain that enables albumin binding (>99%),…

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.

InsuranceTom said:

I went looking for why the dosing schedule is what it is and found that almost every practical question on this board has a pharmacokinetic answer…

This is my experience too, for whatever a second data point is worth. The detail I would add is minor and it is already implied above.

Adding the clinical framing, because it changes how the question reads.

Pharmacist here. I want to add the drug interaction perspective on the pharmacology.

Key points from a pharmacokinetic standpoint:

• GLP-1 agonists delay gastric emptying, which can affect Tmax of co-administered oral medications
• Monitor patients on warfarin (INR), levothyroxine (TSH), and oral contraceptives during dose titration
• The albumin-binding mechanism of semaglutide (C-18 fatty acid linker) gives it the ~168-hour half-life that enables weekly dosing
• Steady state is reached at approximately 4-5 weeks after dose initiation or adjustment

Re: the pharmacology specifically — the pharmacology here is well-characterized and the clinical implications are straightforward.