How long does diphenhydramine stay in your system? i took 100mg before sleep, but 12 hours later, i'm still drowsy. the more dosage, the longer?

Takes a while: It sounds like it is still in your system. The half life varies quite a bit from person to person although some sources will describe a half life of around seven hours. That means that seven hours later you can still have half of the medicine in your body. As you can see, you are probably going to need more time to clear it. Stay safe!