What Drugs do to the Brain
All drugs of abuse, including nicotine, marijuana and cocaine, affect the brain’s limbic system. The limbic system is the "pleasure" part of the brain, as it releases the neurotransmitter dopamine when it experiences something enjoyable. Dopamine creates a feeling of pleasure and it wants to have the same experience again. For example, when you eat ice cream and like it, the limbic system tells you to crave it or want it because it created a feeling of pleasure. When a person uses a drug for the first time, an unnaturally strong feeling of pleasure is experienced. It is natural for the brain to want to have this experience again–this is how addiction begins. And no one knows how many times it takes to become addicted.
When the brain produces a flood of dopamine, which makes the user feel "high," neurons begin to produce less dopamine and reduce the number of dopamine receptors. This leads the drug user to need greater and greater amounts of the drug. This is called building a tolerance. Also, when the neurons stop producing as much dopamine, the brain is left with a lower than normal amount and the user feels extremely down and depressed, thus making the brain crave the substance again and in greater dosages.