Dr.GastroMayo said:

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

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.

Dr.LipidDallas 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…

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.

Dr.RenalNash 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%),…

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.

One thing that is still open after james_edin’s answer:

What did you change at the same time, and can you separate the two now?

Reporting back.

Closing this out: the trough rather than the peak explains the pattern I was seeing on day six, which I had been blaming on the vial.