A prescription says to dilute to 50 mL final volume using a stock of 25 mg/mL to deliver 25 mg. How many mL of stock are required?

• A. 0.5 mL
• B. 1 mL
• C. 2 mL
• D. 5 mL

Answer: (B)

Think in terms of how much drug mass you need and how concentrated your stock is. To deliver 25 mg with a stock that is 25 mg per mL, use volume = dose ÷ concentration = 25 mg ÷ (25 mg/mL) = 1 mL. After taking 1 mL of stock, dilute to a total final volume of 50 mL, which means you’d add 49 mL of diluent. If you used 0.5 mL, you’d deliver 12.5 mg; 2 mL would deliver 50 mg. So the correct stock volume to achieve 25 mg is 1 mL.