If you stayed awake during the second hour of anatomy, you might remember that there's something called the blood-brain barrier. Normally, this nifty anatomical trick keeps your brain safe by filtering out all the harmful stuff that could get into your bloodstream, thus keeping it away from the delicate and mostly-defenseless brain. Unfortunately for us, the blood-brain barrier also keeps things like chemotherapy drugs and antibiotics out of the brain. We occasionally have to stick 'em into the brain directly to see an effect.