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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 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.

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Insulin Day Supply
A patient uses Tresiba FlexTouch (3 mL pen, 100 units/mL) at 30 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 box (5 pens)? Round down to the nearest whole number.
days

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How to set up an insulin day supply

Insulin day supply is a days-supply calculation with one extra rule: an opened vial or pen is only good for a fixed number of days, after which what's left is discarded. So the day supply is the smaller of two numbers, how long the units last and how long the product stays usable once it's in use. Questions on this site give you the in-use stability; on the exam you may be expected to know the common ones.

vial = 10 mL × 100 units/mL = 1,000 units  •  standard pen = 3 mL × 100 units/mL = 300 units, box of 5  •  Toujeo pen = 1.5 mL × 300 units/mL = 450 units, box of 3

day supply = usable units ÷ daily dose, where usable units per container = the smaller of (units in it) and (daily dose × in-use stability in days), rounded DOWN

Worked example 1: a vial that outlasts its stability

A patient uses Humalog (10 mL vial) at 30 units/day. An open vial is stable 28 days.

raw days = 1,000 ÷ 30 = 33.3 days

33 is more than 28, so the last 5 days' worth of insulin is thrown away with the vial. The day supply is 28 days. Had the dose been 40 units/day, 1,000 ÷ 40 = 25 days, under the cap, and the answer would be 25.

Worked example 2: a box of pens, no waste

Lantus SoloStar, box of 5 pens, at 40 units/day. Each pen is stable 28 days once in use; unopened pens stay in the fridge.

units per pen = 3 × 100 = 300 units

days per pen = 300 ÷ 40 = 7.5 days

box = 5 × 300 ÷ 40 = 37.5, round down → 37 days

The stability limit applies to each pen from the day it's opened, not to the box. A pen that lasts 7.5 days never approaches its 28-day limit, so nothing is wasted and the whole box is used. The cap only bites when one pen's units would last longer than its in-use stability.

Worked example 3: pens where the cap bites

Humulin 70/30 KwikPen at 20 units/day. In-use stability is 10 days.

days per pen if fully used = 300 ÷ 20 = 15 days

usable units per pen = 20 × 10 = 200 units

one pen = 200 ÷ 20 = 10 days

box of five = 5 × 200 ÷ 20 = 50 days

Each pen is discarded with 100 units still in it. A box of 5 is therefore a 50-day supply, not the 75 that 1,500 ÷ 20 would give.

Priming and rounding

Pens are primed before each injection, and some questions build the priming into the daily consumption: if the stem says to count 2 units of priming per injection, a 20-unit once-daily dose uses 22 units of pen per day. Apply priming only when the question tells you to; the practice problems here don't. Whatever the setup, the final day supply always rounds down, because a partial day can't be billed.

How to set up an insulin switch

Switching between basal insulins is a dose conversion, and almost every switch is 1:1: the same number of units of the new product as the old one. Two switches are the exception and are reduced by 20%.

NPH twice daily → glargine once daily: 80% of the total daily NPH dose   •   Toujeo (U-300 glargine) → Lantus, Basaglar or Semglee (U-100 glargine): 80% of the Toujeo dose   •   everything else: 1:1

Worked examples

NPH 25 units twice daily to Lantus once daily:

total NPH = 25 × 2 = 50 units/day

Lantus = 50 × 0.8 = 40 units once daily

Toujeo 60 units daily to Basaglar:

Basaglar = 60 × 0.8 = 48 units daily

Lantus 40 units daily to Tresiba: 1:1, so 40 units. Semglee to Lantus, Basaglar to Lantus, Lantus to Toujeo: all 1:1.

The 20% reductions go one way. Lantus to Toujeo is 1:1 (Toujeo is then titrated up as needed), and NPH once daily to glargine is also 1:1; the reduction applies when NPH was being given twice a day. When a question describes a switch in the reduced direction, the total daily dose of the old insulin is what gets multiplied by 0.8, so add up both NPH doses first.

Where students lose the points

10 Insulin Day Supply & Switching practice problems with worked solutions

Every problem below is worked start to finish. Try it yourself first, then check the steps. For a fresh problem every time, use the practice box above. It generates unlimited variations of these same question types.

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
  1. Tresiba → Lantus is a 1:1 conversion (no dose reduction).
  2. New dose = 30 units of Lantus daily.
  3. 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
  1. Units per pen: 1.5 mL × 300 units/mL = 450 units.
  2. 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.
  3. Day supply = 450 units ÷ 15 units/day = 30 → 30 days.
  4. 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
  1. Basaglar → Tresiba is a 1:1 conversion (no dose reduction).
  2. New dose = 64 units of Tresiba daily.
  3. 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
  1. Units in vial: 10 mL × 100 units/mL = 1,000 units.
  2. Raw days: 1,000 units ÷ 30 units/day ≈ 33.3 days.
  3. Lyumjev is stable 28 days once in use. Since 33.3 > 28, the leftover insulin is discarded → cap at 28.
  4. Day supply = 28 days.
  5. 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
  1. Toujeo (U-300) → Basaglar (U-100 glargine) uses a 20% reduction.
  2. 26 units/day × 0.8 = 20.8 units of Basaglar daily.
  3. 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
  1. Units in vial: 10 mL × 100 units/mL = 1,000 units.
  2. Raw days: 1,000 units ÷ 50 units/day = 20 days.
  3. Novolin R is stable 42 days once in use. Since 20 ≤ 42, no waste.
  4. Day supply = 20 days.
  5. 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
  1. Total daily NPH: 26 units/dose × 2 doses/day = 52 units/day.
  2. NPH (BID) → basal glargine uses a 20% reduction: 52 units/day × 0.8 = 41.6 units of Lantus daily.
  3. 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
  1. Units in vial: 10 mL × 100 units/mL = 1,000 units.
  2. Raw days: 1,000 units ÷ 20 units/day = 50 days.
  3. Humalog Mix 50/50 is stable 28 days once in use. Since 50 > 28, the leftover insulin is discarded → cap at 28.
  4. Day supply = 28 days.
  5. 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
  1. Total daily NPH: 18 units/dose × 2 doses/day = 36 units/day.
  2. NPH (BID) → basal glargine uses a 20% reduction: 36 units/day × 0.8 = 28.8 units of Lantus daily.
  3. 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
  1. Units per pen: 3 mL × 100 units/mL = 300 units. Box = 5 pens × 300 units/pen = 1500 units.
  2. 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.
  3. Day supply = 5 pens × 300 units/pen ÷ 25 units/day = 60 → 60 days.
  4. 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.

Sources and how far they go

Where a convention on this page comes from a published standard, the standard is named. Where it comes from a textbook or from ordinary practice instead, the entry says so. That difference matters when a preceptor or an exam disagrees with you.

Citations are to the source of the method. They are not a review of this page, and none of these bodies has reviewed or endorsed it.

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