An animal requires 30 mg of Gentocin IV. Gentocin is available in concentrations of 50 mg/mL and 100 mg/mL. How many milliliters are needed for a 30 mg dose at each concentration?

• A. 3 ml and 6 ml
• B. 6 ml and 3 ml
• C. 0.6 ml and 0.3 ml
• D. 0.3 ml and 0.6 ml

Answer: (C)

When you need a dose in milligrams but the medicine is in a certain mg per mL concentration, use volume = dose ÷ concentration. The 30 mg dose divided by 50 mg/mL gives 0.6 mL, and divided by 100 mg/mL gives 0.3 mL. So you’d administer 0.6 mL at the 50 mg/mL concentration and 0.3 mL at the 100 mg/mL concentration. This also shows that a higher concentration requires a smaller volume to deliver the same dose.