Drug dosage in three steps

Why this matters

You will do these calculations on the licensing exam, and doing them right is critical for the survival of your patients.

By the end of this lesson you will be able to

Identify the three pieces of information every dosage calculation needs

Carry out the two-step conversion from body weight to volume drawn up

You need exactly three things

The weight of the animal in kilograms or grams
A drug dosage for the species, usually written mg/kg or µg/kg
The concentration of the medication you have, usually written mg/mL

Step 1: milligrams neededweight (kg) × dose (mg/kg) = mg of drug kg × mg/kg = mg (the kg units cancel)

Step 2: volume to draw upmg needed ÷ concentration (mg/mL) = mL mg ÷ mg/mL = mL (the mg units cancel)

The Study Guide's worked example

Frank receives a baby opossum caught by a cat. It has a few puncture wounds and is otherwise healthy. His veterinarian advises an antibiotic at 10 mg/kg. The bottle reads 5 mg/mL. The opossum weighs 1 pound.

Weight in kilograms1 lb = 454 g, and 454 g ÷ 1,000 = 0.454 kg

Milligrams needed0.454 kg × 10 mg/kg = 4.54 mg

Volume4.54 mg ÷ 5 mg/mL = 0.9 mL

Frank draws up 0.9 mL and gives it to the opossum.

Do not skip step 2

Drugs come as a liquid, not a powder. Knowing you need 4.54 mg does not tell you how much to put in the syringe. If you have difficulty with dosage calculations, confer with your veterinarian or an experienced rehabilitator before administering any medication.

This lesson is part of the NY Wildlife Rehabilitator Exam course. Open it in the course to answer the retrieval checks that go with it, track what you have read, and quiz the whole unit. Lessons, quizzes and the glossary are free.

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