Accepted answer
Four weeks is a pharmacokinetic floor with a tolerability rationale layered on top, and liraglutide is exactly the tell you thought it was. The rule the schedules encode is do not judge tolerance of a dose until the exposure from that dose has substantially stopped rising, and how long that takes is set entirely by elimination half-life.
The arithmetic
Semaglutide has a terminal half-life of roughly 165 hours, or about 7 days, from fatty-acid acylation and albumin binding. Dose once weekly and the dosing interval equals one half-life, which makes the accumulation maths unusually clean. Take one dose's worth of drug as 1 unit, assume you can ignore the absorption phase, and track the amount in the body:
| Dose number | Amount just after dosing | Amount 7 days later (trough) | Percent of eventual steady-state peak |
|---|
| 1 | 1.000 | 0.500 | 50.0 % |
| 2 | 1.500 | 0.750 | 75.0 % |
| 3 | 1.750 | 0.875 | 87.5 % |
| 4 | 1.875 | 0.938 | 93.8 % |
| 5 | 1.938 | 0.969 | 96.9 % |
| 6 | 1.969 | 0.984 | 98.4 % |
| limit | 2.000 | 1.000 | 100 % |
The general term is fraction of steady state = 1 − 0.5^n after n doses, because each interval is exactly one half-life. So:
- Four doses — one four-week step — puts you at 93.8 % of the exposure that dose will eventually produce.
- Five doses gets you to 96.9 %, which is where the "about five weeks to steady state" figure in the label comes from.
- Two doses would put you at 75 %, meaning a quarter of the eventual exposure has not arrived yet when you make the decision.
Four weeks is therefore the shortest interval at which a tolerance judgement is mostly about the dose you are on rather than about the exposure still accumulating. It is not a round number that happened to be convenient; it is four half-lives.
Why liraglutide steps weekly
Liraglutide's half-life is about 13 hours and it is dosed daily. Dosing interval 24 h, half-life 13 h, so the accumulation ratio is 1/(1 − 2^(−24/13)) = 1/(1 − 0.278) = 1.38 and steady state is reached in roughly three days. A one-week step is already five half-lives past steady state. Same rule, different number, because the rule is expressed in half-lives and the label is expressed in weeks.
This also explains why tirzepatide's four-week step is comfortable rather than marginal. Its half-life is about 5 days, so a 7-day interval is 1.4 half-lives, the accumulation ratio is 1/(1 − 2^(−7/5)) = 1/(1 − 0.379) = 1.61, and four doses reach roughly 98 % of steady state. The same four weeks is generous for tirzepatide and only just adequate for semaglutide. The interval was set by the slowest agent in the class and then reused, which is how conventions form.
The approved ladders
| Agent and indication | Route / frequency | Ladder | Minimum step interval | Label maximum |
|---|
| Semaglutide, weight management | subcutaneous, weekly | 0.25 → 0.5 → 1.0 → 1.7 → 2.4 mg | 4 weeks | 2.4 mg |
| Semaglutide, type 2 diabetes | subcutaneous, weekly | 0.25 → 0.5 → 1.0 → 2.0 mg | 4 weeks | 2.0 mg |
| Semaglutide, oral | oral, daily | 3 → 7 → 14 mg | 30 days | 14 mg |
| Tirzepatide, both indications | subcutaneous, weekly | 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg | 4 weeks | 15 mg |
| Liraglutide, weight management | subcutaneous, daily | 0.6 → 1.2 → 1.8 → 2.4 → 3.0 mg | 1 week | 3.0 mg |
| Liraglutide, type 2 diabetes | subcutaneous, daily | 0.6 → 1.2 → 1.8 mg | 1 week | 1.8 mg |
| Dulaglutide, type 2 diabetes | subcutaneous, weekly | 0.75 → 1.5 → 3.0 → 4.5 mg | 4 weeks | 4.5 mg |
Three structural observations from the table. First, the diabetes and obesity ladders for the same molecule differ — semaglutide tops out at 2.0 mg for glycaemia and 2.4 mg for weight, and liraglutide at 1.8 versus 3.0 mg. The ladder belongs to the indication, not to the molecule. Second, the first rung is always sub-therapeutic on purpose: 0.25 mg semaglutide and 2.5 mg tirzepatide are explicitly described in their labels as initiation doses not intended for glycaemic effect. They exist to expose the gastrointestinal system to the mechanism at low intensity. Third, the increments are roughly geometric, not arithmetic, at the bottom of the ladder and become arithmetic at the top — semaglutide doubles, doubles, then adds 0.7 and 0.7; tirzepatide doubles once then adds 2.5 repeatedly. That shape is what you get when the constraint at low dose is receptor occupancy and the constraint at high dose is tolerability.
The tolerability layer
The pharmacokinetics set the floor; gastrointestinal adaptation sets why anyone bothered. Nausea on this class attenuates over roughly two to four weeks at a constant dose, which is a coincidentally similar timescale, so a four-week step also happens to be about the time needed for the adaptation to occur. In the registration programmes the escalation phase was where nearly all the nausea lived — in the semaglutide 2.4 mg obesity trial, nausea was reported by about 44 % of the active arm against 18 % on placebo, concentrated in escalation and largely transient [1].
None of the above is a recommendation about anybody's schedule; it is an account of how the published schedules were constructed. Dose decisions belong with a clinician who has your history in front of them.
edited 10 Jul 2024 by j_wierzbicki — removed a claim I could not source
The "four weeks is four half-lives" framing is the thing I was missing. Everything else in the table follows from it. – mz_4113 2 months ago 8The indication-owns-the-ladder point deserves more attention. People quote 2.4 mg as if it were a property of the molecule. – Dr_Signe_Baldursdottir 7 days ago 2Accumulation ratio 1.61 for tirzepatide versus 2.00 for semaglutide is a nice compact way to say the same thing. – ten_mg_vial 8 months ago add a comment