Hi Jenna. There is no way of knowing if a medicine is 100% safe to use on humans. In fact I don’t know any medicine that is 100% safe because every human is different and reacts differently to the medicines. The best we can do is to show that a medicine is ‘probably safe’. The best way to do this is to use another mammal. Once we have done this, the medicine will be given to a very few healthy humans at a low dose to see if it is safe, The dose is then increased up to a level that is expected to work as a medicine and given to a few patients in order to see whether it is safe in them and if it does actually work.