Missed a GLP-1 Microdose? What Half-Life and Steady State Mean for the Next Injection

A weekly injection is easy to forget. Travel, a stomach bug, a pharmacy delay, a vial that ran out two days early. The question that follows is always the same: take it now, or wait for the usual day? The answer is written in the prescribing information, and the reason behind the answer is a single number, the half-life.

Semaglutide has an elimination half-life of about 7 days. Tirzepatide's is about 5. Those two figures set almost everything about how the drugs behave over time: how many weeks it takes to reach a stable level, how far a missed dose lets that level fall, how long the labels give you to catch up, and how long the drug lingers after the last injection. A microdose protocol runs on the same clock as a full dose. The amounts are smaller, but the half-life is the same, so the timing rules do not change.

This article walks through the pharmacokinetics in plain terms, models what a missed dose does to blood levels, and lays out what the US labels for semaglutide, tirzepatide, dulaglutide, and liraglutide say to do. It also looks at what splitting a weekly dose does to the peak and trough, since that is the main place microdose schedules differ from the labelled ones. None of it replaces a prescriber's instruction for a specific product and person.

How long do semaglutide and tirzepatide stay in the body?

One number explains the weekly schedule

Semaglutide stays in the body for about 5 weeks after the last injection, and tirzepatide for about 4, because their elimination half-lives are about 7 days and 5 days respectively. A half-life is the time it takes for the concentration in blood to fall by half. After one half-life 50 percent remains, after two 25 percent, after three 12.5 percent, and after five about 3 percent, which is the conventional point at which a drug is considered cleared. The Wegovy label puts the washout at about 5 weeks and the Zepbound label at about 30 days, and both figures are just five half-lives.

The half-life is long because of engineering, not because the peptide itself is stable. Native GLP-1 is broken down by the enzyme DPP-4 within about two minutes. Semaglutide carries an amino acid substitution that blocks DPP-4, and a fatty acid chain that binds tightly to albumin in the blood, which keeps the drug in circulation and out of the kidney's filters. Tirzepatide uses the same albumin binding strategy on a different peptide backbone. Dulaglutide takes another route, fusing the GLP-1 sequence to an antibody fragment, and lands at about 4.7 days. Liraglutide, an older albumin binding design with a weaker fatty acid, manages about 13 hours and needs daily injection. Exenatide, with none of these modifications, lasts about 2.4 hours and was given twice daily.

Peak concentration after a semaglutide injection arrives at 1 to 3 days, and for tirzepatide the label gives a range of 8 to 72 hours. The slow absorption from the subcutaneous depot smooths the peak, so the level rises over the first couple of days, then declines across the rest of the week, and the next injection lands before it has fallen far. That overlap is what produces accumulation, which is the subject of the next section.

How long does it take to reach steady state?

About a month, for both drugs

Semaglutide reaches steady state after 4 to 5 weekly doses and tirzepatide after about 4, according to their labels. Steady state is the point at which the amount cleared during the week equals the amount injected, so the concentration traces the same rise and fall every week. Before that point each dose lands on top of what is left of the previous ones, and the level climbs. With a 7 day half-life, half of each semaglutide dose is still present when the next one is given, so the trough after the first dose is 50 percent of the eventual steady state trough, 75 percent after the second, 87.5 percent after the third, and 94 percent after the fourth. Tirzepatide, with 38 percent of each dose left at the next injection, gets to 98 percent after four.

The steady state level is about double what a single dose produces for semaglutide, and about 1.6 times for tirzepatide. This is the accumulation ratio, and it matters for anyone reasoning about a low dose. A single 0.25 mg semaglutide injection does not represent what 0.25 mg weekly feels like; by week four the body is carrying nearly twice that exposure. It also explains why the labelled escalation steps are four weeks apart. A dose increase does not show its full effect on appetite or on nausea until the new steady state is reached, and stepping up sooner stacks a second increase on top of an incomplete first one.

The practical consequence for a missed dose follows directly. If a single dose delivers half the steady state level, then a single missed dose removes about half of it, and the rebuild takes the same three to four weeks that the original climb did. The next section models that.

What happens to drug levels when one dose is missed?

A two week gap, then a three week rebuild

Missing one weekly dose lets the level fall for 14 days instead of 7, which in a simple model takes the semaglutide trough to about 50 percent of its steady state value and the tirzepatide trough to about 38 percent. That is the low point. Once injections resume, each dose closes about half the gap for semaglutide and about 62 percent of it for tirzepatide, so the trough is back to 75 and 77 percent one week later, 88 and 91 percent after two weeks, and 94 and 97 percent after three. Four resumed doses put both drugs within 3 percent of where they started.

What people notice is hunger, and the timing fits the curve. The first week of the gap is the normal dosing interval, so nothing changes. Appetite tends to return in the second week, as the level drops below the trough the body has adapted to, and it is often strongest in the day or two before the resumed dose takes effect. Gastric emptying speeds back up over the same period, which is why some people find that food sits differently. After the resumed dose the effect returns in a few days, and the small remaining shortfall over the next two weeks is rarely felt.

A microdose protocol sits on the same curve at a lower height. Someone on 0.25 mg semaglutide weekly who misses a dose drops to the same 50 percent of their steady state as someone on 2.4 mg, and rebuilds at the same rate. The absolute change in concentration is smaller, and so is the absolute size of the step when dosing resumes, which is relevant to the side effect question in the next section. What a low dose does not do is shorten the gap or speed the recovery. The clock is set by the half-life, not the dose.

What do the labels say to do after a missed dose?

Take it if you are inside the window, skip it if you are not, never double

The Ozempic label says to take a missed semaglutide dose within 5 days of the scheduled day and then return to the regular day; after 5 days, skip it. The Wegovy label frames the same rule from the other end: take the missed dose if the next scheduled dose is more than 2 days away, and skip it if the next dose is less than 2 days away. The Mounjaro and Zepbound labels give tirzepatide a 4 day, or 96 hour, window, after which the missed dose is skipped. The Trulicity label gives dulaglutide the same 4 days, phrased as taking it if at least 3 days remain until the next scheduled dose. In every case the next dose goes on the usual day at the usual amount, and no label allows two doses to be taken together.

The windows come from the minimum spacing the labels require between doses. Semaglutide doses must be at least 2 days apart, and tirzepatide doses at least 3 days apart, so a catch up dose on day 5 or day 4 still leaves the required gap before the next scheduled one. The same minimums govern changing the injection day. Moving a semaglutide day from Monday to Thursday is fine because 3 days separate them; moving it to Tuesday the same week is not. The simplest way to shift a schedule is to move the day later by a few days, or to move it a day or two at a time across consecutive weeks.

The Wegovy label adds one more instruction that the others state less explicitly: if more than 2 consecutive weekly doses are missed, the prescriber should consider restarting the dose escalation schedule. The reasoning is in the pharmacokinetics. After two missed weeks the semaglutide level has fallen to about 25 percent of steady state, and after three to about 12 percent, which is close to a cold start. A full maintenance dose injected into that state produces a jump in concentration nearly as large as the first dose ever did. The STEP trial post hoc analysis by Wharton and colleagues found that nausea and vomiting clustered around dose escalation steps, when the concentration was rising fastest, rather than at the highest doses per se. A restart from a lower dose recreates the gentle rise that the original escalation was designed to provide. Liraglutide, on a daily schedule, has the same rule at a shorter timescale: after more than 3 days missed, the Saxenda label says to restart at 0.6 mg.

Does splitting a weekly dose change the peak and trough?

The same average, a flatter curve, and no trial to confirm the benefit

Splitting the same weekly amount into two or more smaller injections does not change the average drug level across the week, but it narrows the swing between peak and trough. In a one compartment model, once weekly semaglutide has a trough about 50 percent below its peak, because one half-life passes between injections. Twice weekly halves the interval, and the trough sits about 29 percent below the peak. Every other day brings the swing to about 18 percent, and daily injection to about 9 percent. Tirzepatide swings more at every interval, about 62 percent once weekly and 38 percent twice weekly, because its half-life is shorter relative to a week.

This is the pharmacokinetic argument behind the split dosing schedules that appear in some microdose protocols, where a weekly amount is given as two or three injections. The theory is that side effects such as nausea track the peak and the rate of rise, while hunger returning at the end of the week tracks the trough, so a flatter curve should improve both ends. The theory is plausible, and the Wharton analysis linking nausea to escalation steps points the same way. It has not been tested. No randomised trial has compared split dosing with once weekly dosing of the same total for either drug, and the approved products are labelled for once weekly use only. Compounded products are the only setting in which split dosing is practical, and the dosing error risks of drawing small volumes from multi-dose vials are covered elsewhere on this site.

Split dosing also changes the missed dose arithmetic in a way worth spelling out. Missing one of two twice weekly injections leaves a 7 day gap instead of 3.5, which drops the semaglutide trough to about 71 percent of steady state rather than 50 percent, and the rebuild is faster because the next injection arrives sooner. The flip side is that a split schedule has twice as many injections to remember, and the labels offer no guidance for it. Anyone on a split schedule needs an explicit missed dose rule from whoever wrote the protocol.

How long does it take to wash out after stopping, and what happens to weight?

The drug is gone in five weeks; the weight takes a year to come back

Semaglutide is about 97 percent cleared 5 weeks after the last injection, and tirzepatide reaches the same point in about 4 weeks. The curve is the mirror image of accumulation: 50 percent of semaglutide remains at day 7, 25 percent at day 14, 12.5 percent at day 21, 6 percent at day 28, and 3 percent at day 35. Appetite and gastric emptying return along the same curve, and most people describe the change as gradual over the second and third weeks rather than sudden. The labels use these washout periods for practical decisions: Wegovy is stopped at least 2 months before a planned pregnancy, and the anaesthesia guidance discussed elsewhere on this site is built around how much drug is still present on the day of a procedure.

Weight does not follow the washout curve. In the STEP 1 extension, participants who stopped semaglutide 2.4 mg after 68 weeks regained about two thirds of their lost weight over the following year, and cardiometabolic markers drifted back toward baseline over the same period. In SURMOUNT-4, participants switched from tirzepatide to placebo at week 36 regained about 14 percent of body weight over 52 weeks, while those kept on the drug lost a further 5.5 percent. Regain is slow because it depends on the cumulative surplus of calories eaten once appetite returns, not on the drug level on any given day. Five weeks without drug is a fully washed out body with, typically, a pound or two of regain. A year without drug is where the trials measured the two thirds.

The distinction matters for how a missed dose should be read. A single missed week is a dip, not a stop, and produces neither washout nor meaningful regain. Two or three missed weeks put the body most of the way to washout, which is why the re-escalation rule exists, but still well short of the timescale on which the trials measured regain. Maintenance dosing, covered in its own article, is the question of how low the level can be kept without the curve drifting toward the regain the extension studies describe.

What is worth knowing and raising with a prescriber?

Five points, none of which are a protocol

Everything below is context for a conversation rather than an instruction. The labelled missed dose rules apply to the approved products at their labelled doses, and a compounded or split schedule needs its own rule from the prescriber who wrote it.

What is genuinely unresolved here?

Three gaps worth naming

The models in this article are one compartment simplifications with a population mean half-life. Real semaglutide and tirzepatide curves have an absorption phase that blunts the peaks, and individual half-lives vary with body weight and kidney function. The 50 percent and 38 percent figures for a missed dose are the right order of magnitude, not measurements from anyone's blood. Nobody has published concentration data from people who actually missed doses in the trials, because the trials were designed to prevent that.

The link between a rising concentration and nausea is well supported for dose escalation, but the specific claim that a resumed full dose after a gap reproduces first dose side effects is inferred from that, not tested directly. The re-escalation advice in the Wegovy label is a precaution rather than the result of a trial comparing restart strategies. Whether one step down is enough, or a full restart is needed, is a clinical judgement without controlled evidence behind either answer.

Split dosing is the largest gap. The pharmacokinetic case for a flatter curve is sound arithmetic, and the clinical case that a flatter curve reduces side effects or end of week hunger is a hypothesis. No trial has compared split and once weekly schedules of the same weekly total for either drug, the approved products are not designed for it, and the compounded products that make it practical carry their own accuracy risks. Anyone weighing a split schedule is weighing a plausible model against an absence of data.

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