All drugs has side effects. Some drugs makes you sleepy, others - like for example antidepressants - can give you a headache. Other remedies can make your heart beat fast. Why do drugs have side effects?

Our bodies are complex structures, built from chemicals and to function smoothly it has to be regulated. Chemicals like for instance hormones, enzymes and other molecular messengers normally make these adjustments. The purpose of medicines are often to take the place of one of the body's regulating chemicals, because of a disease that has been brought out of balance and readjust this. And this is what any successful medicine or drug treatment actually does.

To understand why medicine produces side effects, we have to pay attention to the following:

1. Our body often uses the same chemical to regulate more than one process. What this means is that a medicine may retune not only the desired target but also others that don't need readjustment. Let's use prednisone as an example; this drug gets rid of inflammation, but it also causes thinning of bones.

2. Unfortunately, drugs are not always as selective as we would like them to be. A consequence of this is that the medicine may alter a number of unrelated processes at the same time. The antidepressant amitriptyline can help depression but it can also lower blood pressure by affecting norepinephrine receptors, cause blurred vision, dry mouth and constipation by blocking acetylcholine receptors and even induce sleepiness and weight gain by binding to histamine receptors.

3. It is also a fact that two people taking the same medicine can have very different experiences. One person may have severe or troublesome side effects that make the medicine intolerable (like nausea during the first weeks of taking Effexor), while another person hardly notice any side effects at all. This is the reason of the long list of possible side effects that is enclosed in the drug box.

Side effects may be rare or common, serious or simply annoying. Doctors are required to assess the risk of side effects versus the expected benefit of any medication. Statistically most doctors do a good job, so if you have a doctor you feel you can trust in general, there is no need for distrusting his or her abilities to figure out which medication is best for you.