WendyG_ATL 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.

mike.trainer_LA 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%),…

mike.trainer_LA has the substance of this right. The condition it depends on is worth stating. 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.

WendyG_ATL 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…

Mine went the same way, slower. Posting only so the count is not one.

Clinical perspective, offered as context rather than as advice.

PK/PD modeling for the pharmacology: understanding the pharmacokinetics helps optimize dosing. Semaglutide:

• Tmax: 24-72 hours post-injection
• T½: ~168 hours (7 days) — enables weekly dosing
• Steady state: reached at 4-5 weeks
• Bioavailability (SubQ): ~89%
• Volume of distribution: ~12.5L (primarily plasma)

The albumin binding (>99%) is the key pharmacological innovation — creating a sustained-release effect from a single injection. Previous GLP-1 agonists (exenatide) required BID dosing due to rapid clearance.