Holidays, surgery, sickness: interruption in practice

Two questions get merged and should not be. The first is what to do about a single missed weekly dose, which product labelling generally answers with a defined window and an instruction not to double up. The second is what to do after a gap long enough to clear the drug, which labelling addresses briefly if at all. The pharmacokinetics distinguish them cleanly: one omission leaves roughly half of accumulated exposure at the point the next dose was due; a month leaves almost none.

What the withdrawal designs established

Two trials in this class were built specifically to answer what happens when treatment stops. In the semaglutide programme, participants who had escalated to the top dose over twenty weeks were then randomised to continue or to switch to placebo; those continuing lost a further eight per cent of body weight over the following forty-eight weeks while those withdrawn regained about seven per cent.¹ In the tirzepatide programme, a thirty-six-week open-label lead-in was followed by randomised continuation or withdrawal, with the continuation group losing a further five and a half per cent and the withdrawal group regaining approximately fourteen per cent.²

These are among the most informative results in the field and they are frequently over-read. What they establish is that the effect is maintained by continued exposure and reverses without it. What they do not establish, because neither design examined it, is whether a reduced maintenance dose would hold the result. The comparison was full dose against nothing.

Given that cost is the leading reported reason for stopping, a randomised comparison of full-dose against half-dose maintenance would be one of the highest-value trials nobody has run.

A gap, quantified

Residual exposure after an interruption follows from the half-life alone. For a seven-day half-life at steady state, one missed week leaves about half of accumulated concentration at the point the next dose was due, two weeks about a quarter, three weeks about an eighth, four weeks roughly six per cent, and six weeks under two per cent.

That table does most of the practical work. A single omission is a minor perturbation and the labels reflect this, generally permitting the dose to be taken within a defined window and otherwise skipped, with an explicit instruction not to double up. A gap of a fortnight leaves enough drug that resumption at the previous dose is usually uneventful. A gap of a month or more is functionally a fresh start.

The Journal notes that the labels handle the first case clearly and the third case briefly, and that the second case — the two-to-three-week gap that supply problems produce most often — is where guidance is thinnest and where the numbers above are most useful.³

Re-titration in practice

The convention after a long interruption is to resume one or two rungs below the previous dose and re-escalate on the standard interval. Clinicians described re-escalation as generally faster than the original ascent, on the grounds that a person who previously tolerated a rung is likely to tolerate it again, but there is no trial evidence for accelerated re-titration and the kinetic argument for four-week steps applies unchanged.

Two failure modes recur. The first is resuming at the previous top dose because that was the dose on the last prescription, which reproduces first-exposure symptoms in someone who has forgotten what they were like. The second is the opposite: restarting at the initiation dose after a two-week gap, which discards several weeks of adaptation for no reason.

Both are avoidable with the residual-exposure figures and a note of the date of the last injection. The Journal has come to regard that date as the single most useful piece of information a person on this treatment can keep, and the one most reliably absent when it is needed.

Supply interruption as a dosing event

In this market, gaps are usually structural rather than personal. Shortage listings, restrictions on compounded supply, price movements, customs interdiction and vendors ceasing to trade all produce interruptions that arrive without notice and end without warning. We have documented gaps of one to eleven weeks arising purely from supply, in people who missed no dose voluntarily.

The practical consequence is that anyone dependent on a single source is also dependent on that source for the continuity of their titration. Several people described re-escalating three times in a year for reasons that had nothing to do with their tolerance of the drug.

There is a second-order effect worth naming. Resuming with material from a different supplier compounds the uncertainty: the person is re-escalating and simultaneously changing the actual content of the vial. Reports from Janoshik, Medutest, PeptideMeter and VendorInvestigate consistently show that nominal strength and measured peptide content are not the same quantity, and a supplier change during a re-titration makes any symptom change uninterpretable. Change one variable at a time is a laboratory principle, and it applies here.

A dose is a number about a vial

Everything above assumes the dose administered is the dose intended. For licensed pens that assumption is reasonable. For research-grade lyophilised powder it is an assumption that should be examined, because a titration schedule built on an unreliable starting figure propagates the error through every subsequent rung.

Two distinct quantities are involved. Chromatographic purity describes the proportion of peptide-related material that is the intended peptide. Peptide content describes what fraction of the vial mass is peptide at all, the remainder being counter-ions, residual solvent, water and excipient. A vial can be ninety-nine per cent pure and contain substantially less peptide than its label states, and content is the figure that determines a dose.

Of the four independent services this market relies on, all report purity and only some report content routinely. Janoshik, Medutest, PeptideMeter and VendorInvestigate have each published results in which nominal and measured strength diverged. The Journal has argued in Analytics that content should be reported as standard, and we repeat it here for a titration-specific reason: without it, the arithmetic of a step is being performed on a number nobody has measured.

If there is a single practical conclusion here it is that the ladder is a default and the person is the variable. The trials that produced the figures everyone quotes were run on populations permitted to hold, delay and step back, and reading their results as an endorsement of a rigid calendar inverts what actually happened. We will keep making that point until the labels catch up with the protocols.