How Long GLP-1s Stay in Your System

I have come to think that a surprising share of the questions I get about these drugs are the same question.

Why is my bad day Tuesday and not Sunday? I missed a dose on holiday, do I double up? How long before I can try to conceive? I have surgery in three weeks, what do I do? I stopped last month and I still don’t feel hungry — is something wrong?

Every one of those is a half-life question. Once you have the number, you can answer all of them yourself, which is why I would rather teach the number than answer them one at a time.

The number

Two things in that table deserve attention.

The top row is the whole reason these drugs exist. Your own GLP-1 is destroyed by an enzyme called DPP-4 within about two minutes. Semaglutide is that same peptide with two structural edits — one blocking the enzyme’s attack site, one attaching a fatty acid chain that lets it ride on albumin instead of being filtered out. Two minutes becomes one week. I have written about that mechanism at length; the entire innovation is a duration-of-action problem being solved.

Oral semaglutide has the same half-life as the injection. It is taken daily not because it clears faster, but because so little of each tablet is absorbed. Same molecule, same persistence, different absorption problem — which is why tracking it is a different exercise.

How to use a half-life

One rule does almost all the work: after four to five half-lives, a drug is substantially gone — down to around 3–6% of where it started.

For semaglutide, at roughly one week per half-life:

For tirzepatide, the same shape on a five-day clock: about 25% left at ten days, about 6% at twenty, essentially cleared by three to four weeks.

This is why nothing about these drugs happens quickly, in either direction. You are dealing with a molecule that measures its own life in weeks.

The four questions this answers

1. “Why is my worst day Tuesday?”

Because injecting is not the same as peaking. After a subcutaneous semaglutide injection, blood levels typically reach their maximum one to three days later. The drug is still arriving while you are going about your Monday.

That single fact reframes the first three months for most people. Your bad day is not random and it is not a sign something is wrong — it is the concentration curve, and it is reproducible. Two dose cycles of writing it down will show you yours, and then you can move your injection so the difficult day lands mid-week rather than on a Saturday you would like to enjoy.

2. “Why does the new dose take so long to do anything?”

Because of accumulation. When you dose a drug at roughly its own half-life interval, each dose lands on top of what remains of the last one, and the level climbs over several cycles before it plateaus. That plateau is called steady state, and it takes about four to five weeks for semaglutide and about four for tirzepatide.

Which is exactly why the standard titration interval is four weeks. It is not arbitrary — it is the time required for the dose you just started to actually be the dose you are on.

The practical consequence: judging a dose at day ten tells you very little. Neither the benefit nor the side effects have finished arriving. I have watched a lot of people conclude a dose was not working, one week before it would have.

3. “I missed a dose — do I double up?”

No. Never double up.

For once-weekly semaglutide the label is specific: if the next scheduled dose is more than two days away, take the missed dose as soon as you can. If it is less than two days away, skip it and resume your normal schedule.

The half-life explains why this is safe. Missing one weekly dose does not empty you out — you still have roughly half of the previous dose on board, and often more given accumulation. What doubling up does is stack a full dose onto a level that never fell far, and the result is a very unpleasant week.

If you miss more than two consecutive weeks, that is a call to your prescriber rather than a decision to make alone. Tolerance fades, and you may need to restart lower and re-titrate — which is not a setback, it is the same reason you started low originally.

4. “How long before I can try to conceive?”

This is where the arithmetic becomes consequential.

Semaglutide’s labelling recommends discontinuing at least two months before a planned pregnancy. Note that this is considerably longer than the five weeks pure clearance would suggest — the recommendation builds in margin, and I would follow it rather than the arithmetic. Tirzepatide’s washout is shorter but the same principle applies.

Two related points I make sure to raise. These drugs can restore ovulation in people who were not ovulating, particularly with PCOS, so unintended pregnancy is a real scenario rather than a theoretical one. And tirzepatide reduces the absorption of oral contraceptives, with a backup-method window that resets after every dose increase. Both are covered in more detail in GLP-1s, Cycles, and Perimenopause.

Two more places the weeks matter

Procedures and anaesthesia. A drug that persists for weeks and delays gastric emptying means your stomach may not be empty after a standard fast. Tell your anaesthetist, unprompted, in pre-op. Guidance has moved from broad withholding toward risk stratification, clear liquids the day before, and gastric ultrasound where available — but your instruction is the same either way: say it out loud.

Stopping. Appetite does not return the morning after your last injection. It returns gradually, over roughly a month, as the level decays. People find this genuinely disorienting: they stop, feel fine for three weeks, conclude they have escaped the regain data, and are then surprised in week six. Nothing has gone wrong. The drug simply took that long to leave.

Knowing where you are on the curve

Here is where I will make a practical suggestion, with the disclosure at the foot of this piece.

The reason this article exists is that almost nobody knows, at any given moment, where they are on their own concentration curve. It is the number that explains their week, and it is invisible.

The Zenday App plots it. From your logged dose history it shows you how much of the drug is estimated to still be in your system, decaying across the days between injections and accumulating across a titration — so instead of an abstract half-life you have a curve with today marked on it. I have found it changes how people interpret their own week, because a bad Tuesday stops being a mystery when you can see that Tuesday is your peak.

Two honest caveats, because I would say the same about any tool that draws a line.

It is a model, not a measurement. It is calculated from published pharmacokinetics and your dose history, not from your blood. Real clearance varies between people with kidney function, body size, and other factors, so treat it as a well-founded estimate rather than a result.

It is not a dosing instrument. Nothing on that curve should prompt you to change a dose, take an extra one, or skip one. Those decisions belong with your prescriber. What the curve is genuinely good for is understanding — why this week felt like that, why the new dose has not landed yet, and how far through a washout you actually are.

The one-line version

Semaglutide, about a week. Tirzepatide, about five days. Multiply by five for how long until it is gone, divide the interval by nothing at all when you miss a dose, and expect everything — good and bad — to arrive and depart on a scale of weeks rather than days.

Almost every confusing thing about living on these drugs makes sense once you are thinking in half-lives.