A medication comes as 2 mg/mL. If the order is 0.5 mg, what is the volume to administer?

• A. 0.125 mL
• B. 0.50 mL
• C. 0.25 mL
• D. 1.00 mL

Answer: (C)

To find the volume, use volume = dose ÷ concentration. The medication is 2 mg per 1 mL, so to deliver 0.5 mg you need 0.5 mg ÷ (2 mg/mL) = 0.25 mL. Therefore, administer 0.25 mL. This matches a quarter of a milliliter. The other volumes would produce under- or overdoses: 0.125 mL would give 0.25 mg, 0.50 mL would give 1 mg, and 1.00 mL would give 2 mg.