Here's an interesting fact about illicit drugs and misused prescription drugs: All street drugs and many prescription drugs (when abused) act as false neurotransmitters. They enter the brain and cross synapses to stimulate postsynaptic neurons, just like the body's own neurotransmitters; but they overstimulate the "reward system" causing exceessive release of dopamine which eventually dries up, and the user "crashes". Addiction is caused by glutamate, which is caused to "teach" the need for the drug. The matter is, of course, far more complex than the above comments can cover, but what's said here suffices to explain that an old commercial stating "Drugs fry the brain" is pretty accurate.