Provided that the body is regarded as a linear system and the response to a unit impulse input is Riemann integrable in the interval [0, ∞), the validity of Dost's law of corresponding areas is demonstrated. No other restrictions are placed on the nature of the blood drug concentration-time curve. The derivation is independent of any drug distribution models. However, the specification of a drug input point is essential for the application of the area relationship. A limit theorem for the improper integration of a convolution integral is presented.