Whatever the actual mechanism of action of a drug may be, we will want to know: Does the drug actually reach its site of action, and for how long does it stay there? This is governed by three factors:
1. Absorption: Uptake of the drug from the compartment of application into the blood
2. Distribution: Transport / equilibration between the blood and the rest of the organism
3. Elimination: Filtration and secretion in the kidneys; chemical modification in the liver
Broadly speaking, absorption and distribution determine the whether a drug will be available at its target site at all, while elimination determines for how long the drug effect will last. The issues of drug absorption, distribution and elimination are collectively referred to as `pharmacoki-netics'.