Insulin Day Supply & Switching Practice

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Insulin calculations at the pharmacy come down to two things: figuring the day supply of a vial, pen, or box (which depends on the product's in-use stability, not just the total units), and converting between basal products when a patient is switched. Initiation dosing is a clinical decision, not a dispensing calculation, so it isn't tested here.

FormulasVial = 10 mL = 1,000 units • standard pen = 300 units (box of 5) • Toujeo pen = 450 units (box of 3)Day supply = total units ÷ daily dose, capped at in-use stability (rapid & glargine 28 d, Humulin 31, Novolin/Levemir 42, Tresiba/Toujeo 56; mixed pens 10–14 d). Round DOWN.Switching (basal → basal) is 1:1 EXCEPT NPH (BID) → glargine and Toujeo → Lantus/Basaglar, both −20%

Worked example

Lantus 10 mL vial at 25 units/day: 1,000 ÷ 25 = 40 days, but an open glargine vial is stable 28 days → 28-day supply. Switching NPH 30 units BID to glargine: 60 × 0.8 = 48 units daily.

💡 Stability caps the day supply of an open container, and the 20% reduction only applies going NPH→glargine or Toujeo→Lantus/Basaglar — not the reverse.