Hello!

Medicines are made up of certain chemicals which act on specific receptors. And your body tissues have also got certain receptors- cells or groups of cells which are formulated to respond to only a certain type of stimulus(say chemicals). When you take any medicine, it reaches your circulatory system after getting absorbed from your gut. While in blood it travels all around in your body and when it reaches its specific receptor, it binds to the receptor and brings about the desired action.

There is another mechanism also! The other mechanism is centrally mediated by the brain. There are nerves connecting your brain to each and every cell of your body like a huge network of wires. The faulty nerves switch themselves off. And this switch off mechanisms is done with the help of certain drugs which directly act on the central nervous system. Painkillers work usually in the same way.

Hope you understood!