I take a pill for depression but get side effects everywhere. it happens to work in the head but why does it affect those areas?

The whole body: Antidepressants affect the whole body. They definitely target the brain to help lift your depression, but those same receptors are also in other parts of your body. For example, serotonin receptors are in your brain and that is where most antidepressants function, but your gut has more serotonin receptors than your brain. That is why some people have nausea or constipation.