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Split dose calculator

Break a weekly dose into smaller, more frequent injections without changing the weekly total. Space them evenly, every few days, or on fixed days of the week. Then see what it actually does to your peaks and troughs, which is the only reason to bother.

By OptiPin Editorial·Last updated August 28, 2026

Compound (sets the half-life used for the levels below)

Dose you take now

How often now

Split into

Perfectly even spacing, which ignores the calendar. Twice a week means every 3.5 days, so the injection drifts by half a day each week and lands on a different weekday every other shot.

Concentration (optional, to get units per injection)

Each injection becomes

weekly total, unchanged

units per shot (U-100, 100 u = 1 mL)

peak-to-trough swing

Splitting a dose is a protocol change. For prescription medication, including every GLP-1, dividing a weekly dose across more injections is off-label and not what the manufacturer's labelling describes. Discuss it with whoever prescribes for you. The level estimates use a one-compartment model on published half-lives and show shape and scale, not your personal blood levels. Not medical advice.

And some things do not wait. A site that turns red, hot or hard, redness that spreads, or a fever after injecting, needs same-day medical care: more injections means more sites and more entries into the vial. Shakiness, sweating or confusion on a GLP-1, especially alongside insulin or a sulfonylurea, is possible low blood sugar. Take 15 g of fast carbohydrate, then get help if it does not lift.

Every schedule at this weekly total

The same weekly amount, spread differently. Peaks and troughs are the estimated amount in your system once levels have settled, so the useful column is the last one: how far your levels travel between injections.

Schedule	Per injection	Units (U-100)	Peak (mg)	Trough (mg)	Swing

Want the full curve rather than the endpoints? The half-life calculator plots build-up, steady state and washout for the same compound.

Before you change anything

See the new schedule against your real data

OptiPin forecasts your levels from the doses you actually log, calibrates to your bloodwork, and keeps the reminder schedule in step when you change frequency. All on-device.

Download on the App Store

How to use this calculator

Enter what you take now

Your current per-injection dose and how often you take it. The weekly total is what gets held constant.

Pick the new frequency, and how you want to count it

Even spacing divides the week mathematically. Every few days holds a whole-day interval. Set days of the week pins the injections to named days, which is what most people actually do, and then the gaps stop being equal.

Read the swing, not just the dose

Dividing the dose is arithmetic anyone can do. The point of splitting is a flatter curve, and how much flatter depends entirely on the compound's half-life.

Three ways to count the interval

Even spacing is the textbook answer and nobody's real schedule. Twice a week is every 3.5 days, which means 8am Monday, 8pm Thursday, 8am Monday. Perfectly flat on paper, and a schedule that alternates morning and evening forever. It is the right number to compare against, and the wrong one to plan your week around.

A whole-day interval keeps the time of day fixed and lets the weekday drift. Every 3 days is always the same hour, but it walks around the calendar: Monday, Thursday, Sunday, Wednesday. Only 1 and 7 divide the week cleanly, so anything else trades a stable weekday for a stable gap.

Fixed weekdays keep the calendar and give up equal gaps. Monday and Thursday are 3 days apart one way and 4 the other. Splitting a 3.5 mg week into 1.75 mg twice leaves the 4-day stretch slightly short and the 3-day one slightly heavy, which is exactly the small sawtooth people notice at the end of the longer gap.

So the doses follow the gaps, not the injection count. Monday's dose has to carry 3 days and Thursday's has to carry 4, so it is 3/7 and 4/7 of the week: 1.5 mg on Monday and 2 mg on Thursday. That is not a rounding artefact, it is the point. If you would rather draw the same number twice, switch to Same dose each time and the calculator shows you what that costs in swing.

Two days apart is not the same as twice a week. Monday and Tuesday is technically twice a week and pharmacologically a once-weekly dose with a stutter. The chart below compares your chosen days against every even schedule at the same weekly total, so you can see how much of the flattening survives the day you actually picked.

When splitting actually helps

It depends on the half-life, not on preference. If the interval is short relative to the half-life, levels barely move between injections and splitting changes almost nothing. If the interval is long relative to the half-life, each dose largely clears before the next one lands, and splitting is the difference between a spike and a plateau.

Short esters gain the most. Testosterone propionate on a weekly schedule is a peak followed by several days of very little. The same weekly total every other day is a far steadier line. Cypionate and enanthate gain much less from going past twice weekly, which is why most TRT protocols stop there.

For GLP-1s the motive is usually tolerance, not smoothness. Semaglutide is close to a week and tirzepatide is nearer five days, so weekly dosing is flatter than a testosterone ester. Flatter is not flat: run the numbers above and both still show a meaningful drop in peak-to-trough swing when they are split. Whether that is worth a second injection is a question about tolerance rather than about the curve.

More injections is a real cost. Every extra injection is another site, another entry into the vial and another chance to draw the wrong number. If the swing barely improves, the flatter curve is not worth it.

Splitting does not change your weekly exposure. The area under the curve is the same. If you are splitting in the hope of taking less, you are not; you are taking the same amount differently.

Common questions

How do I split a weekly dose into two injections?

Halve the per-injection amount and keep the weekly total the same. A 2.5 mg weekly dose becomes 1.25 mg twice a week if the two injections are evenly spaced, which means 3.5 days apart rather than two days running.

Should Monday and Thursday get the same dose?

Not if you want even levels. Monday to Thursday is 3 days and Thursday to Monday is 4, so the Thursday dose has a longer stretch to cover. Splitting by the gaps gives it 4/7 of the week and Monday 3/7, which turns a 3.5 mg week into 1.5 mg on Monday and 2 mg on Thursday. Dosing both the same is simpler to draw and slightly less even.

Can I inject every 3 days instead of twice a week?

Yes, and the gaps are then genuinely equal, which twice-a-week on fixed days never is. The cost is that 3 does not divide into 7, so the injection day moves through the week: Monday, Thursday, Sunday, Wednesday, and so on. Pick whichever you are more likely to actually follow.

Does splitting a dose reduce side effects?

It lowers the peak, and where a side effect tracks the peak rather than the average, a lower peak can help. It does not reduce your weekly exposure at all.

Is splitting GLP-1 doses safe?

It is off-label. Manufacturer labelling for semaglutide and tirzepatide describes once-weekly dosing, and splitting is not a studied schedule. It is also mechanically riskier because each draw is smaller.

How do I know if splitting is worth it for my compound?

Compare the swing figures. If going from weekly to twice weekly barely moves it, the compound's half-life is already long relative to your interval and you are adding injections for very little.

Do I need to change my dose when I start splitting?

Not the weekly total. Splitting is a redistribution rather than a titration, and changing both at once leaves you unable to tell which change caused what.