A medication is 4 mg/mL. If the prescribed dose is 9 mg, how many milliliters should you administer?

A medication is 4 mg/mL. If the prescribed dose is 9 mg, how many milliliters should you administer?

• A. 1.5 mL
• B. 3.0 mL
• C. 4.5 mL
• D. 2.25 mL

Answer: (D)

Dividing the prescribed dose by the concentration tells you how much liquid to give. Since the dose is in milligrams and the strength is in milligrams per milliliter, the milligrams cancel, leaving milliliters. So for 9 mg at 4 mg/mL: 9 ÷ 4 = 2.25 mL. Administer 2.25 milliliters. Quick check: 2.25 mL × 4 mg/mL = 9 mg, confirms the calculation.

Dividing the prescribed dose by the concentration tells you how much liquid to give. Since the dose is in milligrams and the strength is in milligrams per milliliter, the milligrams cancel, leaving milliliters. So for 9 mg at 4 mg/mL: 9 ÷ 4 = 2.25 mL. Administer 2.25 milliliters. Quick check: 2.25 mL × 4 mg/mL = 9 mg, confirms the calculation.