Drugs like marijuana and heroin work by turning on neurons because their properties work like a neurotransmitter, they can even trick receptors and work as nerve cells. The problem with this is that they send out abnormal messages "messing up" the body.
Drugs like amphetamine cause cells to produce an abnormal amount of neurotransmitters as well as preventing the recycling of brain chemicals. This causes an over exaggerated message sent out to the body which greatly wears the communication channels down. It could be compared to the difference between someone whispering in your ear and screaming into the microphone.
The one thing that all drugs have in common, be it nicotine, cocaine, marijuana, and methamphetamine, is that they all reward the brains limbic system. When the brain responds to feelings of pleasure it releases dopamine into the blood system. Drugs cause dopamine to flood the system. The flood of domamine produces the "high" and is highly addictive.
