Various routes are used to administer drugs, but in most cases drugs reach their site of action via the systemic circulation. Once within the circulation a drug is clearly not confined to its intended site of action. Instead it is distributed widely throughout the body.
At this point you may have anticipated some important questions:
How is the drug carried in the blood?
Is the entire drug free to exert an effect?
Can the presence of one drug in the circulation affect another?
To begin to understand the answer to these questions we need first of all to think about plasma proteins.