Drug circulation in the bloodstream

Once they have reached the bloodstream, some drugs bind tightly to proteins in the blood such as albumin. In this form, the drugs are usually inactive and are quickly removed from the body. If a drug is highly protein-bound in this way, the dose is carefully chosen so that a large enough proportion of each dose remains unbound and therefore active in the body.

Some drugs are far more protein-bound than others. If you take two highly protein-bound drugs for example Drug X and Drug Y, Drug X may bind to most of the available protein in the blood, leaving little protein left to bind to Drug Y. That can mean that an unusually high proportion of each dose of Drug Y is left unbound and active, perhaps the equivalent of taking an overdose of Drug Y.