This is the first in a series of posts featuring stuff from the real world (for varying values of "real") that you can use in your games either as a player or a GM.
Mindhacks has a link to an article discussing various ways to get past the brain-blood barrier.
Not only is the brain-blood barrier a cool phrase, it's also the way things get from the bloodstream to your brain. The methods listed (such as implantable devices) are very cyberpunk but they could be used in any setting that has a reason to put something into someone's brain for good or ill.
Take it a step past drugs and you could implant memories, emotions, or even erase them with these methods.