Empathize with him.  Tell him that you know that his illness is extremely difficult to understand and deal with, that you want him to get better, and specificly tell him that you hate to see him suffer.  The conversation should focus on him and helping him and not about how frustrating it is for everyone else, no ultimatums.    He may really believe that his medication is poisonous or harmful and there is no point in trying to convince him otherwise.  Try to avoid arguing about it.  The best bet is to lead him to believe that it is worth a try, that he will have to stay on the medication for a while to see if it will stop the suffering.  You could mention that it is better to deal with it now than later.   I know this is very frustrating!  I'm dealing with this issue with my father for about the thousandth time in my life.  I often wonder how different his life would have been if he was born during my generation.    I really hope that your stepson responds well to the medication.  Good luck.