Semaglutide half-life: where the 165 to 184 hour figure comes from posts 121–146

Narrowing post #120, because the general version has more than one answer.

Semaglutide's structure is a modified backbone with a C18 diacid attached through a spacer. The acylation is the reason for the long half-life and it is also the reason a plain sequence comparison against native GLP-1 is misleading about how the molecule behaves.

The 165 to 184 hour half-life range gets quoted as a fixed number. It is a range across a studied population, and individual clearance varies enough that a person's own steady state may be reached earlier or later than the population average implies.

An honest declaration on semaglutide half-life: I have a prior here and it is strong enough that you should weight what I say downward. Stating it rather than hiding it.

The mass figure differs between sources because of how the different entities report it. The monoisotopic mass of the bare peptide is one number; the mass including the acylation is another. Different sources emphasise different details.

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

Semaglutide versus liraglutide in STEP 8: semaglutide produced greater weight reduction and the discontinuation rates differed. But the trial was open-label for the dosing schedule, which admits expectation effects. Weekly versus daily itself is part of the comparison, not a confounding variable to be removed.

Reading it again, the caveat matters more than the finding.

Posting my semaglutide half-life numbers with the method attached so they can be discounted properly. Uncontrolled, unblinded, and collected by someone who wanted a particular answer.

On alcohol: the labelling does not contraindicate it, but it raises gastric irritation risk and semaglutide already does that. There is no published interaction study and the conservative position is to limit it if you are titrating or if gastrointestinal symptoms are troublesome.

I am confident about the direction and much less about the magnitude.

Worth separating two things that post #126 runs together.

The C-cell question: semaglutide triggered medullary thyroid carcinoma in rodent toxicology studies. A signal in rodents does not automatically appear in humans, but it is the reason the compound is contraindicated in people with personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2.

That is clearer than the version I had in my head. Thank you.

The 165 to 184 hour half-life range gets quoted as a fixed number. It is a range across a studied population, and individual clearance varies enough that a person's own steady state may be reached earlier or later than the population average implies.

Adding the measurement that post #131 says would settle it.

Injection site does not appear to matter much for semaglutide exposure. The published comparisons of abdomen, thigh and upper arm found differences small enough to be clinically unimportant, which is not true of every injectable.

I had written a reply contradicting post #131 and deleted it. Here is what survived.

Reframing semaglutide half-life slightly, because I think the disagreement is about the question rather than the answer. If the question is "does it happen", yes. If it is "how often", nobody here knows.

Semaglutide's structure is a modified backbone with a C18 diacid attached through a spacer. The acylation is the reason for the long half-life and it is also the reason a plain sequence comparison against native GLP-1 is misleading about how the molecule behaves.

I would treat the number as indicative rather than as a measurement.