Split-Dose & Half-Life Simulator

How this tool works · sources read October 2, 2026 · Grey Peptides Editorial Board

What the simulator shows

The simulator takes one weekly amount and spreads it four ways: all at once every seven days, half every three and a half days, an equal share every other day, or a seventh every day. Each schedule delivers the same total over a week. What changes is the shape of the curve: how high the amount in the body climbs after each dose, how far it falls before the next one, and how long it takes for the build-up to level off. Those three things, peak, trough and time to steady state, are what people mean when they argue about splitting a weekly injection into smaller, more frequent ones.

The vertical axis is a relative amount, where 100 is one full weekly dose absorbed. It is not a blood concentration, it carries no milligrams, and nothing on the page is a dose. Each label specifies its own schedule, and the label schedule is the one that was tested.

The model, in plain terms

Each curve is a one-compartment model with first-order absorption, the standard textbook description for a drug injected under the skin. After a dose, the drug moves from the injection site into the body at a rate set by its absorption constant, and leaves the body at a rate set by its elimination half-life. A single dose therefore rises to a peak and then falls by half every half-life. Repeated doses add together, and the total keeps rising until the amount eliminated between doses equals the amount given; that balance is steady state.

Two numbers drive everything, and both come from the drug's US label: the elimination half-life and the time to peak after an injection. The absorption constant is not stated on labels, so the simulator solves for the value that makes a single dose peak exactly at the label's time to peak. Where a label gives a range, the model uses its midpoint, the same choice our half-life visualizer makes, and the compound list and its values are shared with that tool, which checks them against our encyclopedia entries.

Reading the three numbers

Peak-to-trough ratio at steady state is the highest modeled amount in a dosing interval divided by the lowest, once the build-up has levelled off. A ratio of 2 means the amount doubles from its low point to its high point every interval; a ratio near 1 means it barely moves. Splitting the same weekly amount into smaller, more frequent doses always pulls this ratio toward 1, because each dose is smaller and the gap in which it can fall is shorter.

Accumulation ratio is how much higher the steady-state level is than the level after the first dose of that same schedule. It is calculated per schedule, so a daily schedule shows a much larger number than a weekly one simply because each daily dose is a seventh of the size; it does not mean the daily schedule ends higher overall. For the same weekly total and complete absorption, the average steady-state amount is the same whatever the schedule.

Time to 95% of steady state depends almost entirely on the half-life. A drug reaches about 94% of steady state after four half-lives and 97% after five, whatever the dosing interval, which is why splitting a dose does not make a long-acting drug reach its plateau much sooner.

Checking the model against the labels

A model is only worth showing if it reproduces what the labels report. The page's builder refuses to publish unless three label statements come out of the model. For tirzepatide, the Mounjaro and Zepbound labels state that steady-state concentrations were achieved after 4 weeks of once-weekly dosing, with a half-life of about 5 days ^{1 2}; the model reaches 86% of steady state after two weekly doses and 98% after four. For dulaglutide, the Trulicity label gives an accumulation ratio of about 1.56 at steady state, steady state between 2 and 4 weeks, and a half-life of about 5 days ³; the model gives an accumulation ratio of 1.61 and reaches 95% of steady state after four weekly doses. For semaglutide, the Wegovy label says that with a half-life of about one week it will be present in the circulation for about 5 to 7 weeks after the last dose ⁴; the model leaves 3.1% after five weeks and 0.8% after seven.

These checks also falsify: give dulaglutide a 200-hour half-life, tirzepatide 300 hours or semaglutide 80, and the build fails. The browser code and the builder's Python run the same equations, and the build compares them for every compound.

What splitting does, and does not, do

Run semaglutide's label values and the pattern is clear. Once weekly, the modeled amount swings by a factor of about 1.6 between peak and trough at steady state; twice weekly, about 1.2; daily, almost flat. People who split weekly injections usually do it to soften the peak, on the theory that side effects such as nausea track the high point. The model shows that splitting does flatten the curve. It cannot show whether that changes side effects, appetite or weight, because no label and no trial we know of tested split schedules of these drugs against the labelled weekly schedule.

Splitting also has costs the model leaves out. More injections mean more chances for error, and a pen designed to deliver a fixed weekly dose cannot deliver half of one reliably; our guide to GLP-1 pens and vials explains why no label describes partial doses. Daily drugs work the other way round: liraglutide, with a half-life of about 13 hours and a peak 8 to 12 hours after the dose, is labelled for once-daily use ⁵, and stretching it to every other day would deepen the troughs considerably.

Missed doses

Choose a week in the missed-dose menu and the doses due in that week are removed. For a long-acting drug the curve sags gradually rather than collapsing: with semaglutide's one-week half-life, the amount keeps falling by half each week, so a week after the skipped dose it is about half its usual low point, and the curve takes several weeks of regular dosing to climb back. Shorter-acting drugs fall faster and recover faster. Each label has its own instructions for what to do after a missed dose, and those instructions, not this model, are what a patient should follow.

What the model leaves out

Real pharmacokinetics are more complicated than one compartment. The model assumes every dose is completely absorbed at the same rate, that elimination is the same at every dose level, and that every person has the label's average half-life. Labels report variation between people, differences with kidney function and body weight, and the effect of where the injection is given, none of which is modeled. The amount plotted is in the whole body, not in the blood, and it is relative to one weekly dose rather than in milligrams. Use it to understand why schedules behave as they do, and use the label and a prescriber for anything about an actual dose.

Sources

1. Eli Lilly. Mounjaro (tirzepatide) US prescribing information, section 12.3 (DailyMed set d2d7da5d-ad07-4228-955f-cf7e355c8cc0, effective August 27, 2026); read October 2, 2026.
2. Eli Lilly. Zepbound (tirzepatide) US prescribing information, section 12.3 (DailyMed set 487cd7e7-434c-4925-99fa-aa80b1cc776b, effective August 28, 2026); read October 2, 2026.
3. Eli Lilly. Trulicity (dulaglutide) US prescribing information, section 12.3 (DailyMed set 463050bd-2b1c-40f5-b3c3-0a04bb433309, effective June 16, 2026); read October 2, 2026.
4. Novo Nordisk. Wegovy (semaglutide) US prescribing information, section 12.3 (DailyMed set ee06186f-2aa3-4990-a760-757579d8f77b, effective June 18, 2026); read October 2, 2026.
5. Novo Nordisk. Victoza (liraglutide) US prescribing information, section 12.3 (DailyMed set 5a9ef4ea-c76a-4d34-a604-27c5b505f5a4, effective October 14, 2025); read October 2, 2026.

This explainer describes how the tool works and what its numbers rest on. It is not medical advice and does not recommend any dose or product.