The health care provider prescribes a one-time dose of diphenhydramine 40mg stat. The pharmacy supplies diphenhydramine 12.5 mg/5mL. How many mL's should the nurse administer? Record the answer as a whole number.

To calculate the correct dose, use the standard dosage formula: volume to administer equals desired dose divided by dose on hand, multiplied by the volume on hand. The desired dose is 40 mg, the dose on hand is 12.5 mg, and the volume on hand is 5 mL. Setting up the equation: 40 mg ÷ 12.5 mg × 5 mL = 3.2 × 5 mL = 16 mL. Therefore, the nurse should administer 16 mL of diphenhydramine 12.5 mg/5 mL to deliver the prescribed 40 mg dose. It is important to double-check this calculation because diphenhydramine is commonly administered in pediatric settings and an error in volume could lead to underdosing or overdosing. The keyed answer of 16 mL is correct. A common mistake would be dividing 40 by 5 to get 8 mL, which fails to account for the concentration of 12.5 mg per 5 mL rather than 1 mg per mL. Another error would be rounding incorrectly; however, in this case the arithmetic yields a whole number, so no rounding is needed, and the answer of 16 mL is exact.