Coming back to: Half-life, steady state, and accumulation worked through

On the subject in the title: Half-life, steady state, and accumulation worked through Working notes rather than a conclusion.

Working through the kinetics rather than the pharmacology, because I think the confusion here is arithmetic.

With a half-life of roughly a week, steady state is approached over four to five half-lives, so anything measured before about a month is measuring a rising concentration. Accumulation at weekly dosing lands the steady-state level near double the first-dose level.

Have I got that right, and does it change what people are actually asking?

Answering the question the opening post raises rather than the one it answers.

Where the Half-life discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.

A pharmacokinetic model fitted to trial data describes the population studied. Applying it to somebody outside the enrolled range is an extrapolation, and the model will not tell you it is.

Where I would look next, rather than where I would stop.

Albumin binding: semaglutide binds albumin through a fatty side chain, which sequesters the free form and extends the half-life. Tirzepatide also has albumin binding (different mechanism) which extends its half-life compared to an unmodified peptide.

That much is documented. The rest is how I have interpreted it.

Useful. I had the fact and not the reason, which turns out to be the important half.

Everything in post #4 holds. The case it does not cover is the one I have.

A note on how Half-life gets discussed rather than on Half-life itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.

Terminal half-life estimated from a short sampling window underestimates the true value. That is a common source of discrepant figures between sources.

That is the honest state of it as of this week.

Washout after stopping takes roughly the same four to five half-lives as reaching steady state. A month after the last dose is not the same as none.

I checked the source rather than the summary, and they differ.

Half-life: semaglutide ≈ 165–184 hours (about a week). Tirzepatide ≈ 5 days. Liraglutide ≈ 13 hours. The half-life determines how much accumulation happens at steady state and how long it takes to clear after stopping.

Nothing above should be read as advice about what anyone else should do.

Accumulation at steady state: with a week-long half-life, steady-state concentration is reached around 4 to 5 half-lives (about 4 to 5 weeks). Before that, concentration is rising with each dose. The clinical implication: escalating before 4 weeks means escalating before steady state.

The disagreement above is smaller than it looks once the terms are fixed.

Post #7 describes the usual case. This is about the unusual one.

What I can speak to on Half-life is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know.

Adding the measurement that post #11 says would settle it.

Summarising the Half-life thread so far, since it is long and the answer is buried: the first reply has the method, the fourth has the correction to it, and the rest is people agreeing at length.

Loading doses are not used in this class and the pharmacokinetic reason is tolerability rather than efficacy. A loading dose would reach steady state faster and would be intolerable.

I have written this out at length because the short version keeps being misread.

Subcutaneous absorption is the rate-limiting step for most of these compounds, which is why the apparent half-life is absorption-limited rather than elimination-limited.

Written quickly, so the reasoning may be tighter than the wording.