Thomas A. M. Kramer, MD

Disclosures

Medscape General Medicine. 2003;5(1) 

In This Article

Introduction

There are two broad sets of issues that we deal with in psychopharmacology: pharmacokinetics and pharmacodynamics. Pharmacokinetics could be defined as what the body does to drugs, and encompasses all of the issues of drug interactions and drug clearance metabolism. Pharmacodynamics could be defined as what the drug does to the body. One could argue that it is the most important group of issues, because it encompasses the actual therapeutic effect of the drug.

Drugs have effects on the body. That is why we give them to patients. Our goal is to have the effect of the drug be therapeutic. Generally speaking, drugs have two kinds of effects: therapeutic effects and side effects. The ideal drug for any therapeutic indication would have only therapeutic effects and no side effects. We all know that in the real world, there are no such perfect drugs, but the best and most commonly used medications have a very high "therapeutic to side effect ratio."

It is, to a certain extent, a value judgment, however, whether a particular effect of the drug is a side effect or a therapeutic effect. Similar to the rallying cry of the software engineer -- "it's not a bug, it's a feature" -- sometimes things that start out being referred to as side effects end up being a primary therapeutic effect. This happens when drugs that were developed for one particular indication end up getting used for something quite different. One of the best examples of this is minoxidil, which was launched in 1979 as a treatment for hypertension. It was noted that many of the patients taking minoxidil who were bald started to grow hair back. Nine years later, minoxidil cream was marketed as treatment for hair loss, and that has become the primary use of this drug. Sildenafil is an enormously successful treatment for erectile dysfunction but was originally developed as an antiangina drug. On clinical trials, it was not particularly successful for that indication, but many of the subjects in the study reported dramatically improved erections.

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