Clinical perspective, offered as context rather than as advice. The version of this that has an answer is narrower than the version being asked. Narrow it and it becomes tractable; leave it broad and the thread will produce nine confident and incompatible replies.

LabKate said:

Practical: unopened, refrigerated at 2 to 8°C; in use, refrigerated and used within the preservative-limited window; never frozen; and a visual check…

Coming at LabKate’s question from a different direction. Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then measure again under the same conditions.

Adding the numbers, since they settle part of this. The mechanism that matters here is not stomach emptying, it is central. GLP-1 receptor agonism in the arcuate nucleus stimulates POMC neurons and suppresses AgRP/NPY signalling, which is why the effect is appetite and food salience rather than physical fullness. Delayed gastric emptying largely tachyphylaxes over the first months; the appetite effect does not.

A narrower follow-up, since the general answer is now clear:

How much of the between-person variation is pharmacokinetic and how much is just adherence measured badly?

Moderator note: leaving this open. It is being argued well and the disagreement is the useful part. Tagging this one for the weekly digest.