My understanding of Half-life is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it.
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Pharmacology · Pharmacokinetics · continued
Half-life, steady state, and accumulation worked through — a second dataset posts 91–120
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
b.restrepo, post #39: Confirming post #38 from a second method, which matters more than confirming it from a second person. Where two sources give different half-lives, check the study design before deciding either is wrong. Sampling duration, assay sensitivity and population all move the number. The variance between people here is larger than the effect… Go to post
On post #90 — agreed on the reasoning, with one qualification.
An observation about Half-life that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.
Collapsed as off-topic by two members at trust level 3 or above
Post #92 is right about the mechanism and I think understates the practical bit.
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.
Take it as a starting point and not as a specification.
Terminal half-life estimated from a short sampling window underestimates the true value. That is a common source of discrepant figures between sources.
I would call that likely rather than established.
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no.silva, post #30: Useful. I had the fact and not the reason, which turns out to be the important half. Go to post
Practical experience of Half-life, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.
Adding thanks rather than a view. I do not have a view worth the space.
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I had written a reply contradicting post #94 and deleted it. Here is what survived.
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.
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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.
Two sources, same conclusion, and I could not rule out that one copied the other.
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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.
It cost nothing to check and would have cost something not to.
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system_suitability, post #19: Adding the measurement that post #16 says would settle it. 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. Go to post
Speaking only to Half-life as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.
d.ferreira, post #42: If you are new and reading this thread for the answer to Half-life: the answer is conditional, the conditions are in the third reply, and the rest of the thread is worth skipping. Go to post
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.
The arithmetic in post #102 is right; the assumption feeding it is the part to check.
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.
Written from notes rather than memory, which is why the numbers are specific.
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Answering the question post #100 raises rather than the one it answers.
Renal clearance: these compounds are cleared partly via the kidney. In severe renal impairment, clearance is slowed and accumulation risk is higher. Dose adjustments might be needed.
I have separated what I observed from what I concluded, which does not always happen.
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That reframing is the whole thing. The facts I already had.
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pe.onwuka, post #33: I will take the caveat as seriously as the claim, which is the point of putting it there. Go to post
I changed my mind about Half-life after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.
Everything in post #104 holds. The case it does not cover is the one I have.
Half-life determines how quickly concentration approaches steady state and does not determine what the steady-state concentration is. Dose and clearance determine that.
That matches what I was told, which is not the same as knowing it.
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Worth separating two things that post #108 runs together.
The most useful thing anyone has posted about Half-life in this category was a table of what had been measured and by whom. That is what I would want again.
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Steady state is approached in roughly four to five half-lives. For a compound with a week-long half-life that is four to five weeks, which is where the escalation interval in the trials comes from.
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Nicolaides, post #93: Post #92 is right about the mechanism and I think understates the practical bit. 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. Take it as a starting point and not as a specification. Go to post
Confirming post #111 from a second method, which matters more than confirming it from a second person.
Renal clearance: these compounds are cleared partly via the kidney. In severe renal impairment, clearance is slowed and accumulation risk is higher. Dose adjustments might be needed.
Where two sources give different half-lives, check the study design before deciding either is wrong. Sampling duration, assay sensitivity and population all move the number.
I would put the burden of proof on the interesting explanation, not the dull one.
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Collapsed as off-topic by two members at trust level 3 or above
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.
That is what the documentation says. What happens in practice is usually close.
s.grigorescu, post #59: Narrowing post #57, because the general version has more than one answer. Terminal half-life estimated from a short sampling window underestimates the true value. That is a common source of discrepant figures between sources. I have left out the parts I could not verify. Go to post
Answering the question post #116 raises rather than the one it answers.
I have three months of notes on Half-life and the honest summary is that the trend is real and the week-to-week numbers are noise. I nearly drew the opposite conclusion from the first fortnight.
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.
The reasoning is more useful than the number, which is why I have shown it.
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The documentation on Half-life is better than this thread and I say that as someone who has posted in the thread.
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Where two sources give different half-lives, check the study design before deciding either is wrong. Sampling duration, assay sensitivity and population all move the number.
That distinction has done more work for me than anything else in this category.
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