What Size?

I needed to replace the lightbulbs in a bathroom fixture. As I stood at the vast lightbulb display, holding the old bulb in my hand, I realized that my choice was limited to two. Nothing else would fit. A couple of years ago I needed to replace the refrigerator in my kitchen. With that purchase I had only one option; I bought the model that fit into the space.

We face much of the same dilemma in clinical medicine. Pharmaceuticals come in defined doses and we need to choose from what is available. Sophisticated delivery systems (sustained release, enteric coating) mean that cutting pills to achieve an in-between dose will affect how a medication is metabolized. Only rarely should capsules be opened.

The good news is that a limited number of dosing options will meet almost every need. Even though patients come in all sizes, healthy kidneys and livers are remarkably uniform in size and function. Lean body mass, which is most often how we determine the amount of medication to use, doesn’t vary too much across the population.

When I bought my lightbulbs, I was confident that the size of the socket was not going to change while I was at the store. Nor is the size likely to change one year from now. It doesn’t matter how often I flip the light switch. Human behavior is not as predictable. Choosing a medication dose for a patient assumes that the medication will be taken the same time every day under the same conditions (with or without food). Additional medications or certain foods can alter drug metabolism. The underlying disease might change. What would appear to be the perfect calculated dose might not work out. This is why lightbulbs are over the counter and most medications need a prescription.