What the pharmacokinetics permit and what they cannot tell you

A substantial informal practice has grown up around non-standard schedules: injecting fortnightly instead of weekly, taking a defined break every few months, dropping to a low dose over the summer and escalating before winter, or stopping and restarting around holidays and supply gaps. The Journal has heard all of these described in reader correspondence, some of them from people under clinical supervision. The randomised evidence for any of them, at any dose, in any population, is as far as we can establish nonexistent.

The physiology was described before the drugs existed

The homeostatic response to weight loss was characterised in a study that predates this drug class and remains the clearest account of it. After a substantial diet-induced reduction, circulating leptin fell and remained suppressed, ghrelin rose and remained elevated, satiety hormones including peptide YY and cholecystokinin fell, and subjective hunger was significantly greater than baseline — and all of these persisted at twelve months, long after active dieting had ended.¹

Alongside the hormonal changes sits a reduction in resting energy expenditure larger than the loss of metabolically active tissue predicts, an effect usually termed adaptive thermogenesis. Its magnitude and persistence are contested, but in the most extreme documented cohort — participants in a televised competitive weight-loss programme followed for six years — resting metabolic rate remained substantially suppressed relative to prediction long after most of the weight had returned.²

Both findings explain regain after drug withdrawal without invoking anything specific to incretins. A drug that suppresses appetite is holding a system away from a defended state. Remove the drug and the system, which has been signalling for restoration throughout, gets what it has been asking for. That is not a drug effect; it is the condition the drug was treating becoming visible again.

Intermittent schedules: the evidence, stated in full

This section is short because the evidence is. The Journal has searched the trial registries and the published literature for any randomised comparison of an intermittent schedule against a standard weekly schedule for any GLP-1 receptor agonist or dual agonist, at any dose, for any indication. We have found none. We have also found no observational cohort large enough to characterise outcomes on such a schedule with the standard confounders addressed.

What exists is dose-ranging data from the phase 2 programmes, which establishes that lower average exposures produce smaller weight effects, and pharmacokinetic modelling, which establishes what average exposure and what peak-to-trough ratio a given interval would produce. Neither tells you whether a fortnightly schedule maintains weight in somebody who has already lost it, which is the question actually being asked.

An absence of evidence is not evidence of harm and the Journal does not present it as such. It is, however, the entire evidentiary position, and readers encountering confident protocols for intermittent use should know that the confidence is not coming from data. Nothing in this section is advice, and the compounds sold for research use only that appear in some of these protocols are not approved for human use.

The arithmetic of a fortnightly schedule

For a drug eliminated with first-order kinetics, the accumulation ratio at steady state is approximately one divided by one minus the exponential of minus the elimination rate constant times the dosing interval. For a seven-day half-life given weekly, that yields a ratio of about two. Given fortnightly, the interval is two half-lives, the residual fraction at the next dose is a quarter, and the accumulation ratio falls to about one and a third.

Two consequences follow. Average concentration on a fortnightly schedule at the same nominal dose is roughly a third lower than weekly, not half, because accumulation differs. And the peak-to-trough ratio rises from modest to fourfold, so the exposure pattern is qualitatively different: the person spends part of each cycle at an exposure that would be sub-therapeutic on a weekly schedule and part at a higher peak.

What the arithmetic cannot tell you is whether that pattern is better, worse or equivalent for maintaining weight, because the relationship between exposure pattern and weight effect is not known — only the relationship between average exposure and weight effect at steady state. The modelling is solid and it answers a different question from the one people bring to it.

Resuming after an interruption is a fresh escalation

This is the most practically consequential item in the whole subject and the one least often stated in advance. Gastrointestinal tolerability to these agents develops over weeks of continued exposure and decays when exposure is removed. After four weeks without the drug, plasma concentrations are a small fraction of steady state and the tolerability accommodation has substantially reset. Resuming at the previous maintenance dose therefore presents the system with an exposure step it has not experienced for a month.

The clinical convention — resume at a lower dose and re-escalate — follows from the pharmacokinetics rather than from caution.³ Product labelling for several agents in the class advises consideration of re-initiation at a lower dose after an extended interruption, and the threshold at which this applies differs between products, which is a detail worth checking against the specific label rather than a general rule.

The shortage period demonstrated the consequence of ignoring this at scale. Large numbers of people lost access for six to ten weeks, resumed where they had left off, and experienced nausea and vomiting considerably worse than during their original escalation. It was predictable, it was predicted by anybody who had read the label carefully, and it was almost never communicated.

What would settle the maintenance question

The Journal’s position is that three trials would resolve almost everything currently argued about in this area, and that all three are straightforward. The first is a dose-reduction design: after a lead-in to target, randomise to full dose, one step down, two steps down, or placebo, and follow for a year with weight as the primary endpoint. It would establish the shape of the descending dose-response curve and would cost a fraction of a pivotal programme.

The second is an interval design: after a lead-in, randomise to weekly, fortnightly and three-weekly administration at the same nominal dose. It would answer the intermittent-schedule question directly and would settle whether the exposure pattern matters independently of average exposure.

The third is a taper design: randomise abrupt cessation against a stepped reduction over twelve weeks, with appetite, eating behaviour and weight measured for a year afterwards. It would test the only argument for tapering that is worth testing.

None of the three is under way as far as the Journal can establish. Readers who know otherwise should write to letters@compoundjournal.com; a registered protocol for any of them would be news in this department.

How the Journal reports a regain figure

Four things accompany every regain number in these pages. Which withdrawal design it comes from, because an off-treatment extension and a randomised placebo switch are different experiments. Whether the lifestyle intervention continued in the arm being described. What the denominator is — regain as a percentage of body weight, as a percentage of the weight lost, or as a final position relative to original baseline, three quantities that are routinely quoted interchangeably. And the follow-up duration, because the regain curve decelerates and a figure at six months is not a figure at a year.

The third of those is where most of the misreporting happens. A statement that participants regained two-thirds is a proportion of loss; a statement that they regained eleven per cent is a proportion of body weight; a statement that they finished 5.6 per cent below baseline is a final position. All three can describe the same arm and they are not interchangeable.

Where a source we are quoting has not stated its denominator, we say that rather than inferring it. Readers who find a regain figure in these pages without its design and its denominator have found an error, and the standards desk would like to hear about it at standards@compoundjournal.com.

What this piece is not

This is reporting on a body of trial evidence and it is not advice about whether or how to stop taking a medicine. The decision to discontinue an agent prescribed for glycaemic control, cardiovascular risk or kidney disease is materially different from the decision to discontinue one prescribed for weight, and in every case it belongs with a clinician who has seen the person and knows why the drug was started.

Two further notes. Compounds sold for research use only are not approved for human use in any jurisdiction, and nothing here should be read as guidance about using them or about stopping their use. And where this piece describes what clinicians report doing about maintenance dosing, that is description of practice and not a schedule anybody should adopt from a magazine.

The Journal takes correspondence on this subject at letters@compoundjournal.com and factual challenges at standards@compoundjournal.com. Letters describing a personal experience of stopping are read with attention and are published, where they are published, as accounts rather than as evidence — a distinction this department tries hard to preserve in both directions.

The stopping question has a clear answer and an unclear set of consequences. Treatment maintains weight loss while it continues; stopping abruptly returns most but not all of the lost weight within a year; and nothing in the randomised record supports the claim that a person ends up worse off than they started. What is genuinely unresolved is everything between full dose and nothing, which is where almost every real decision is made.