Microdosing tirzepatide: the arithmetic, and the measuring problem

"Microdosing" means taking less than the 2.5 mg starting dose — 0.5, 1, 1.25 mg — usually for maintenance after reaching a goal, or because 2.5 was too much. It isn't on the label and there's no trial behind it. If your prescriber has put you there, here's what those doses are in units, and why the small ones are the part to be careful with.

This page does arithmetic. It isn't a recommendation to dose below the label, and it's written for people whose prescriber has already made that call. If that's not you, the standard schedule is the page you want.

The doses, in units

Units on a U-100 insulin syringe. Red: under 4 units — a draw small enough that one mark is a large slice of the dose.
Dose	5 mg/mL	10 mg/mL	20 mg/mL	40 mg/mL
0.5 mg	10	5	2.5	1.25
0.625 mg	12.5	6.25	3.13	1.56
1 mg	20	10	5	2.5
1.25 mg	25	12.5	6.25	3.13
1.5 mg	30	15	7.5	3.75
2 mg	40	20	10	5
2.5 mg	50	25	12.5	6.25

The measuring problem

On a 30-unit syringe the marks are one unit apart, and a careful person lands within about half a mark. At 12.5 units, half a mark is a 4 % error — nobody would notice. At 3.1 units, the same half a mark is 16 %. At 1.5 units it's a third of the dose. The drug hasn't changed; the percentage has.

1.25 mg at 10 mg/mL → 12.5 units

1.25 mg at 20 mg/mL → 6.25 units

1.25 mg at 40 mg/mL → 3.13 units

So the rule for small doses is the opposite of the rule for large ones: you want the weakest vial you can get. 1.25 mg at 10 mg/mL is a comfortable 12.5 units. The same 1.25 mg at 40 mg/mL is a 3-unit sliver barely off the hub. If your pharmacy sends a strong vial for a microdose, ask for a weaker one, or for their written instructions on diluting — not a guess at three units.

What people mean by it

Version	What's going on
Maintenance below 2.5 mg	Reached goal, want to hold it, don't want the full dose or the full price. The most common meaning. No published trial has tested it; what exists is clinician reports and surveys, which is thin.
Starting below 2.5 mg	For people who couldn't tolerate the labelled start. A prescriber's judgement call — the same logic as the in-between doses on the chart.
Stretching the interval	Every 10 or 14 days instead of 7. Not a units question, but worth knowing that tirzepatide's half-life is about five days, so by day 10 the level has fallen a long way and appetite tends to come back before the next dose.

Questions worth asking your prescriber

"Which concentration should this be, so the draw isn't tiny?" — the question this page exists to make you ask.
"Is this a dose or an interval?" — they solve different problems.
"How do we know it's working?" — a microdose that isn't holding your weight is just a smaller bill.

Your vial

Tirzepatide microdose → unitsU-100 syringe

Vial strength mg

Water you added mL

Your dose mg

Syringe

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Fill in all three boxes.

Common setups:

The standard ladder, 2.5 mg and up: the tirzepatide chart. The concentration pages show which vial is comfortable at which dose: 5 mg/mL · 10 mg/mL · 20 mg/mL · 40 mg/mL.