Then you do a Phase III trial, in hundreds or thousands of patients, randomised, blinded, comparing your drug against placebo or a comparable treatment, and collect much more data on efficacy and safety. You might need to do a few of these, and then you can apply for a licence to sell your drug. After it goes to market, you should be doing more trials, and other people will probably do trials and other studies on your drug too; and hopefully everyone will keep their eyes open for any previously unnoticed side-effects, ideally reporting them using the Yellow Card system
