The cold remedy you take makes you sleepy. The antidepressant gives you a headache. The sinus decongestant makes your heart race. Why are side effects so common?
To understand this you need to realize that your body is an immensely complex structure built from chemicals that must be regulated in order to function smoothly. Chemicals such as hormones and other molecular messengers usually make these adjustments. Medicines often work by taking the place of one of the body’s regulating chemicals to readjust the balance. When this restores functioning it is seen as helpful.
However, two things complicate the picture. First, the body often uses the same chemical to regulate more than one process. This means that a medicine may retune not only the desired target but also others that don’t need readjustment. An example of this is that the drug prednisone turns off inflammation, but also causes thinning of bones. Secondly, medicines are not always as selective as we would wish. This means the medicine may alter a number of unrelated processes at the same time. For example, the antidepressant drug amitriptyline can help depression (by acting on serotonin receptors), can also lower blood pressure (by affecting norepinephrine receptors), cause blurred vision, dry mouth and constipation (by blocking acetylcholine receptors) and produce sleepiness and weight gain (by binding to histamine receptors).
One of the more surprising aspects of medications is how two people taking the same medicine can have such different experiences. One person may have severe or troublesome side effects that make the medicine intolerable, while another person finds that the medicine does only the good that it is intended to do. In fact when many thousands or millions of people use medicines, the list of observed side effects can become long indeed.
Side effects may be rare or common, serious or merely annoying. A medicine with frequent mild side effects may be tolerated by the majority of people and be regarded as relatively safe. On the other hand if a medicine has a less common but more serious side effect it can mean that the medicine should only be used when there is no alternative, and then with close monitoring. These considerations require doctors to assess the risk of side effects versus the expected benefit of any medication. In a life-threatening disease, even serious side effects may be worth the risk; but for a mild, transient illness, little risk or even discomfort should be tolerated.
As we learn more about the way our bodies are regulated, more medications are developed that allow us to intercede when diseases disrupt our functioning. It has been a prominent goal in the development of new medications to avoid more serious side effects. But it is likely that until we can foretell who is especially susceptible, at least some people may experience side effects from otherwise beneficial medications. Many of us may have to keep going back to our doctors to try a second or even a third medication before we find one with the strongest therapeutic effect and the fewest side effects.