How does medicine know where to go?

DEAR DR. ROACH >> My older friends and I talk about your column once in a while, and we all read it every day. Thank you. One thing we all wonder is, how do pills work? How does the medicine know where to go?

— D.A.

DEAR READER >> With a few exceptions, the medicine in the pills goes nearly everywhere in the body, but only “work” in specific locations. For example, if you take the blood pressure medicine atenolol, the medicine is absorbed in the small intestine, and then goes everywhere the blood flows. (A few medicines are metabolized by the liver, which complicates things. That’s why estrogen taken by mouth has different risks from estrogen patches absorbed on skin.)

Atenolol is a beta blocker — it prevents the beta1 receptor from being stimulated by epinephrine and similar substances. This has the result of a lower blood pressure, a slower heart rate and less forceful contractions of the heart because of the beta1 receptors in the heart. This makes the medicine very useful in people with some types of heart disease.

However, there are other places that the body has beta receptors. In the pancreas, a different beta receptor — the beta-2 receptor — is used to stimulate insulin release. Atenolol (which blocks both kinds of beta receptors) can decrease insulin release, leading to higher blood sugars in some people. Beta receptors are also in the airways; blocking these can lead to airway constriction and wheezing. That’s partly why beta blockers are used cautiously in people with diabetes and asthma.

Drugs that “work” in multiple locations may have multiple beneficial effects or, more likely, a combination of beneficial and unwanted effects. Pharmaceutical companies have developed medicines that act on very specific receptors, leading to fewer side effects. For ex- ample, the alpha blockers that we use for men with enlarged prostate glands relax the smooth mus- cle in the prostate. The old ones we used to use caused terrible lightheadedness upon standing in many men. The newer ones are much less likely to do that, since they are specific for the exact receptor in the prostate. Even though they travel throughout the whole body, they (hopefully) only have significant effect on the prostate.