I fell downstairs and landed on my back, and now it's very painful, i struggle to bend and stand up, what effective painkiller should i take?

Back trauma: You want to make sure that there is nothing surgical going on your back. While it is entirely possible that all you have is a muscle strain and bruising there is no way to know for sure without having a thorough examination. If you are having pain which is traveling and your legs or if you have had loss of control of bowel or bladder function you need to be evaluated immediately.