Molecular mass of semaglutide and why the figure differs between sources — does this still hold? posts 91–103

One more thing on Molecular mass of semaglutide that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.

On the "does it stop working" question: tolerance in the pharmacological sense is not what the withdrawal trials show. What they show is that the effect persists while treatment continues and reverses when it stops, which is a different finding with different implications.

The interesting part of this is the exception, and I do not understand the exception.

I read post #89 twice before replying, because I had assumed the opposite.

On the injection-day question: the labelling for licensed semaglutide allows the day to be changed provided at least two days separate the two injections. That interval is not arbitrary — it is what stops two doses stacking inside one absorption window.

Worth one more sentence than it usually gets.

Post #93 answers the question as asked. The question underneath it is different.

The STEP programme populations were selected: enrollment criteria required baseline body mass index over 30, no recent blood pressure crisis, no recent retinopathy, no renal disease at the time. Applying results from that population to someone well outside it is an extrapolation and should be called one.

Since Molecular mass of semaglutide keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.

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.

On reflection I would soften that slightly.

Coming back to post #93, because the follow-up matters more than the original answer.

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.

That is the distinction I keep failing to hold on to. Written down now.

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.

That is where I would start, not where I would stop.

Building on post #97 rather than restating it.

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.

This is the version I would want a new member to read first.

Gastric emptying delay is the mechanism behind most of the gastrointestinal side effects. Slowing the stomach empties it feeds less food into the intestines at a time, which is part of the satiety mechanism but is also why nausea is dose-dependent and titration-dependent.

Seconded. It reads as careful rather than confident, which is the right register.

On post #101 — agreed on the reasoning, with one qualification.

Solutions of semaglutide can look faintly opalescent without anything being wrong. Visible particulate, fibres or frank cloudiness are a different observation entirely and are worth raising with the supplier rather than reasoning about.

One more caveat and then I will stop qualifying: the sample selected itself.