Factors contributing to variability in drug pharmacokinetics. II. Drug distribution.

Many factors determine the distribution of a drug within the body. All factors may be altered in physiological and pathological states, and this may result in a need to change either the dose of the drug administered or the dosage interval. Changes in plasma protein binding may also alter the interpretation of the relationship between total plasma drug concentration and drug effect, and in such circumstances the free drug concentration may provide a better guide to optimal therapy.