Drugs are chemicals. They are like all other chemicals except that humans make a value judgement that a particular chemical (drug) will effect a desired change in a living organism. However, it’s important to note that a drug does what it does and there is no perfectly specific drug. So, drug effects always represent a mosaic of chemical interaction and effects. Each drug has properties that we humans hope will bring benefit to other humans. Those are the desired effects, but every drug produces effects other than the desired effect. Those are called side effects. But the drug doesn’t care – it just does what it does. In fact, often, what is a desired effect in one therapeutic setting, may be a side effect in another.
As pharmacologists and physicians interested in treating patients with drugs, we are interested in several properties that all drugs have. These include the mechanisms by which the desired objective is induced, pharmacodynamics, the mechanisms by which the drugs may induce side effects, toxicology, and the mechanisms by which the body distributes and clears a drug from the body, pharmacokinetics. Put simply, we are interested both in what the drug does to the body and what the body does to the drug.
Because drugs are chemicals and chemical reactions depend on collisions between two or more chemicals, drug effects are concentration dependent. We adjust the concentration of a drug in the body by adjusting the dose. All effects of drugs, with the possible exception of allergic reactions, are dose dependent. Therefore, we are interested in the doses that produce a desired effect and the doses that may produce undesired effects, or adverse events (side effects). A drug that produces a desired effect at a much lower dose than the dose required to produce an adverse event is usually considered a better drug. When we assess the difference between the dose that produces a desired effect and a dose that produces an adverse event, we are now analyzing a drug performance in a sophisticated way, and we are thinking about the term therapeutic index. This is the key parameter that you should always be thinking about when you use a drug.