Volume of Distribution
Apparent volume into which a drug distributes — a larger Vd means the drug leaves the bloodstream and enters tissues.
Formula
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Example
500 mg IV → C₀ = 10 mg/L → Vd = 50 L
How it works
The volume of distribution is a way to describe how widely a drug appears to spread through the body after an IV bolus. It is calculated by dividing the amount of drug given by the initial plasma concentration, because if the same dose produces a very low blood concentration, that implies much of the drug has moved out of the bloodstream into tissues. It is an "apparent" volume, not a real anatomical space, and it helps compare drugs with different patterns of tissue binding and fluid distribution.
When to use it
Use this formula when you know the IV bolus dose and the initial plasma concentration soon after administration, and you want to estimate how extensively a drug distributes in the body. It is commonly used in pharmacokinetics to compare drugs or to help interpret whether a drug stays mainly in blood or enters tissues.
Worked example, step by step
Step 1: Identify the values D = 300 mg C₀ = 15 mg/L Step 2: Apply the formula Vd = D / C₀ Vd = 300 mg / 15 mg/L Step 3: Calculate Vd = 20 L Final answer: The volume of distribution is 20 L.
Common mistakes to avoid
- Using this formula for an oral dose instead of an IV bolus
- Mixing up units, such as using mg with µg/L without converting first
- Treating volume of distribution as a literal physical volume rather than an apparent one
Did You Know?
Chloroquine has a Vd of over 200 L/kg — far exceeding total body water — because it concentrates heavily in tissues.
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