Why does benadryl (diphenhydramine) causes drowsiness?

Enters the brain: The molecular structure of benedryl allows it to enter the brain where it can interfere with the chemicals that transfer messages between brain cells. In most people it causes drowsiness, in some it will cause the opposite. Newer antihistamines like loritadine & fexofinadine have molecular structures that prevent entry into the brain, so they don't make you drousy.