Pharmacokinetics describes how drugs are absorbed, distributed and excreted by the body and pharmacodynamics how they act within it. What follows is a simple introduction to some of the (italicised) terms and concepts most frequently encountered.

Drugs taken by mouth are only effective if absorbed, unless, like Gaviscon® or nystatin, they act on, or in, the gut. Many antibiotics are destroyed when given by mouth, although a small alteration in structure may change a drug like benzylpenicillin (penicillin G), which is destroyed by acid, into a drug like penicillin V which is not. Food may reduce intestinal absorption; milk, for example, reduces the absorption of tetracycline. Delayed gastric emptying, poor peristalsis, or ileus will delay arrival in the upper small intestine, where most absorption occurs. Some drugs (like aciclovir) are never completely absorbed. Others, though well absorbed, also show reduced bioavailability because they are metabolised by the liver before reaching the rest of the body, thus showing extensive first-pass