Insulin Day Supply & Switching Practice
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Insulin calculations at the pharmacy come down to two things: day supply, and switching between basal products. Day supply depends on the product's in-use stability once opened, not just the total units in the vial, pen or box. 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 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 exampleLantus 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.
10 Insulin Day Supply & Switching practice problems with worked solutions
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Problem 1: Insulin Switch
A patient takes Tresiba 30 units daily. Convert to Lantus. What is the new daily dose? Round your answer to the nearest whole number.
Answer 30 units
Solution- Tresiba → Lantus is a 1:1 conversion (no dose reduction).
- New dose = 30 units of Lantus daily.
- Rounded as the question asks: 30 units.
Most basal-to-basal switches are 1:1. Only NPH (BID) → glargine and Toujeo → Lantus/Basaglar take a 20% reduction, and those go one direction only.
Problem 2: Insulin Day Supply
A patient uses Toujeo SoloStar (1.5 mL pen, 300 units/mL) at 15 units/day. Once a pen is in use it is stable for 56 days and is then discarded; unopened pens are refrigerated and remain stable. What is the day supply of one pen? Round down to the nearest whole number.
Answer 30 days
Solution- Units per pen: 1.5 mL × 300 units/mL = 450 units.
- Toujeo SoloStar is stable 56 days once a pen is opened; one pen at 15 units/day lasts 450 ÷ 15 = 30 days → within stability, no waste.
- Day supply = 450 units ÷ 15 units/day = 30 → 30 days.
- Rounded as the question asks: 30 days.
Standard pens = 3 mL / 300 units, box of 5. Toujeo pen = 1.5 mL at U-300 = 450 units, box of 3. In-use stability caps a single open pen; unopened pens keep in the fridge, so a box can exceed the per-pen limit.
Problem 3: Insulin Switch
A patient takes Basaglar 64 units daily. Convert to Tresiba. What is the new daily dose? Round your answer to the nearest whole number.
Answer 64 units
Solution- Basaglar → Tresiba is a 1:1 conversion (no dose reduction).
- New dose = 64 units of Tresiba daily.
- Rounded as the question asks: 64 units.
Most basal-to-basal switches are 1:1. Only NPH (BID) → glargine and Toujeo → Lantus/Basaglar take a 20% reduction, and those go one direction only.
Problem 4: Insulin Day Supply
A patient uses Lyumjev (10 mL vial, 100 units/mL) at 30 units/day. Once in use, this product is stable for 28 days, after which any remaining insulin must be discarded. What is the day supply of one vial? Round down to the nearest whole number.
Answer 28 days
Solution- Units in vial: 10 mL × 100 units/mL = 1,000 units.
- Raw days: 1,000 units ÷ 30 units/day ≈ 33.3 days.
- Lyumjev is stable 28 days once in use. Since 33.3 > 28, the leftover insulin is discarded → cap at 28.
- Day supply = 28 days.
- Rounded as the question asks: 28 days.
Insulin day supply = total units ÷ daily dose, then capped at the product's in-use stability. You can never dispense a partial day, so this always goes DOWN.
Problem 5: Insulin Switch
A patient takes Toujeo (glargine U-300) 26 units daily. Convert to Basaglar. What is the new daily dose? Round your answer to the nearest whole number.
Answer 21 units
Solution- Toujeo (U-300) → Basaglar (U-100 glargine) uses a 20% reduction.
- 26 units/day × 0.8 = 20.8 units of Basaglar daily.
- Rounded as the question asks: 21 units.
Switches are 1:1 EXCEPT two cases at −20%: NPH (BID) → basal glargine, and Toujeo (U-300) → Lantus/Basaglar.
Problem 6: Insulin Day Supply
A patient uses Novolin R (10 mL vial, 100 units/mL) at 50 units/day. Once in use, this product is stable for 42 days, after which any remaining insulin must be discarded. What is the day supply of one vial? Round down to the nearest whole number.
Answer 20 days
Solution- Units in vial: 10 mL × 100 units/mL = 1,000 units.
- Raw days: 1,000 units ÷ 50 units/day = 20 days.
- Novolin R is stable 42 days once in use. Since 20 ≤ 42, no waste.
- Day supply = 20 days.
- Rounded as the question asks: 20 days.
Insulin day supply = total units ÷ daily dose, then capped at the product's in-use stability. You can never dispense a partial day, so this always goes DOWN.
Problem 7: Insulin Switch
A patient takes NPH 26 units BID. Convert to Lantus (glargine) once daily. What is the new total daily dose? Round your answer to the nearest whole number.
Answer 42 units
Solution- Total daily NPH: 26 units/dose × 2 doses/day = 52 units/day.
- NPH (BID) → basal glargine uses a 20% reduction: 52 units/day × 0.8 = 41.6 units of Lantus daily.
- Rounded as the question asks: 42 units.
Switches are 1:1 EXCEPT two cases at −20%: NPH (BID) → basal glargine, and Toujeo (U-300) → Lantus/Basaglar.
Problem 8: Insulin Day Supply
A patient uses Humalog Mix 50/50 (10 mL vial, 100 units/mL) at 20 units/day. Once in use, this product is stable for 28 days, after which any remaining insulin must be discarded. What is the day supply of one vial? Round down to the nearest whole number.
Answer 28 days
Solution- Units in vial: 10 mL × 100 units/mL = 1,000 units.
- Raw days: 1,000 units ÷ 20 units/day = 50 days.
- Humalog Mix 50/50 is stable 28 days once in use. Since 50 > 28, the leftover insulin is discarded → cap at 28.
- Day supply = 28 days.
- Rounded as the question asks: 28 days.
Insulin day supply = total units ÷ daily dose, then capped at the product's in-use stability. You can never dispense a partial day, so this always goes DOWN.
Problem 9: Insulin Switch
A patient takes NPH 18 units BID. Convert to Lantus (glargine) once daily. What is the new total daily dose? Round your answer to the nearest whole number.
Answer 29 units
Solution- Total daily NPH: 18 units/dose × 2 doses/day = 36 units/day.
- NPH (BID) → basal glargine uses a 20% reduction: 36 units/day × 0.8 = 28.8 units of Lantus daily.
- Rounded as the question asks: 29 units.
Switches are 1:1 EXCEPT two cases at −20%: NPH (BID) → basal glargine, and Toujeo (U-300) → Lantus/Basaglar.
Problem 10: Insulin Day Supply
A patient uses Humalog KwikPen (3 mL pen, 100 units/mL) at 25 units/day. Once a pen is in use it is stable for 28 days and is then discarded; unopened pens are refrigerated and remain stable. What is the day supply of one box (5 pens)? Round down to the nearest whole number.
Answer 60 days
Solution- Units per pen: 3 mL × 100 units/mL = 300 units. Box = 5 pens × 300 units/pen = 1500 units.
- Humalog KwikPen is stable 28 days once a pen is opened; one pen at 25 units/day lasts 300 ÷ 25 = 12 days → within stability, no waste.
- Day supply = 5 pens × 300 units/pen ÷ 25 units/day = 60 → 60 days.
- Rounded as the question asks: 60 days.
Standard pens = 3 mL / 300 units, box of 5. Toujeo pen = 1.5 mL at U-300 = 450 units, box of 3. In-use stability caps a single open pen; unopened pens keep in the fridge, so a box can exceed the per-pen limit.