A drug is 0.8 mg/mL. The order is 6 mg. How many mL are required?

• A. 6 mL
• B. 7.5 mL
• C. 4 mL
• D. 9 mL

Answer: (B)

To figure out the volume, use the concentration: 0.8 mg per 1 mL. Each milliliter contains 0.8 mg, so to deliver 6 mg you need 6 ÷ 0.8 = 7.5 mL. Checking: 7.5 mL × 0.8 mg/mL = 6 mg, which matches the ordered dose. Other volumes would give more or less drug than prescribed: 6 mL would deliver 4.8 mg, 4 mL would deliver 3.2 mg, and 9 mL would deliver 7.2 mg. Therefore, 7.5 mL is the correct volume.