Following administration, drugs are broken down, absorbed, distributed, and cleared from the body through numerous processes and chemical reactions described in more detail below. Pharmacokinetics is the term used to describe the way in which drugs are dealt with by the human body.

The amount of drug that is absorbed into the body may depend on a range of factors including:

• The dose of drug.
• Consumption of food, drink, and other drugs
• Speed of chemical reactions
• Variation between different individuals

It is important that when drugs are taken, the amount of drug that is active in the body is high enough to have the desired affect, but low enough to limit dose related side effects. Studies of anti-HIV drugs have shown that low blood levels of certain drugs have been predictive of poor long-term outcome and virological rebound as the amount of drug in the blood is not sufficient to fight infection. Similarly, high blood levels of a drug may result in the increased incidence of side effects of that drug.