Dr. Sandson, Director of Education and Training at Sheppard Pratt Hospital, has centered his book on the cytochrome P450 system and other enzymes found primarily in the liver, with small concentrations in the intestinal wall and other tissues. Their important role is to help rid the body of toxins, including the medications we psychiatrists go to so much trouble to introduce. Through a process of oxidative metabolism and conjugation, these enzymes render lipid-soluble drugs water soluble so that they can be excreted through the kidneys. Not all of the P450 enzymes act on all drugs, but each acts on many. When one enzyme is acting on two or more concomitantly prescribed medications at the same time, conditions are ripe for an unanticipated drug interaction. These interactions can include one drug’s slowing another’s excretion, causing it to accumulate until it reaches the clinical equivalent of an overdose, or the first drug can expedite excretion of another, reducing its therapeutic action.