Overdose - Large doses of opiates inhibit respiratory centers in the brain stem. With high levels of opiates the automatic process of breathing is lost. Overall, stimulants increase the amount of excitatory neurotransmitters in the brain which causes a higher frequency of neural impulses. Finally, there is also an increased amount of serotonin in the brain. This is a wide variety of chemicals with varying molecular structures . . .
Most hallucinogens inhibit serotonin neurons in the brain stem.
Typically, serotonin is involved in sensory perception . The effect of hallucinogens is to reduce this filtering process. This is when smaller doses of the drug produce the same effects previously caused by larger doses.