A nurse is preparing to administer phenytoin 75 mg PO every 6 hours. The medication is available as 25 mg per 5 mL. How many milliliters should be given per dose?

A nurse is preparing to administer phenytoin 75 mg PO every 6 hours. The medication is available as 25 mg per 5 mL. How many milliliters should be given per dose?

• A. 10 mL
• B. 15 mL
• C. 20 mL
• D. 25 mL

Answer: (B)

Focus on converting the prescribed milligrams into a volume using the given concentration. The solution contains 25 mg in 5 mL, which means it has 5 mg of phenytoin per 1 mL. To deliver 75 mg, you need 75 mg ÷ 5 mg/mL = 15 mL. So, each dose should be 15 mL. (The dosing interval doesn’t change the volume per dose; it only repeats this amount every 6 hours.) If you multiply across using dimensional analysis: 75 mg × (5 mL / 25 mg) = 15 mL.

Focus on converting the prescribed milligrams into a volume using the given concentration. The solution contains 25 mg in 5 mL, which means it has 5 mg of phenytoin per 1 mL. To deliver 75 mg, you need 75 mg ÷ 5 mg/mL = 15 mL. So, each dose should be 15 mL. (The dosing interval doesn’t change the volume per dose; it only repeats this amount every 6 hours.) If you multiply across using dimensional analysis: 75 mg × (5 mL / 25 mg) = 15 mL.