nicotine;

Quote Originally Posted by wikipedia
It also increases the heart rate and blood pressure and reduces the appetite. In large doses it may cause vomiting and nausea.

In addition, nicotine increases dopamine levels in the reward circuits of the brain. Studies have shown that smoking tobacco inhibits monoamine oxidase (MAO), an enzyme responsible for breaking down monoaminergic neurotransmitters such as dopamine, in the brain. It is currently believed that nicotine by itself does not inhibit the production of monoamine oxidase (MAO), but that other ingredients in inhaled tobacco smoke are believed to be responsible for this activity. In this way, it generates feelings of pleasure. This reaction is similar to that caused by cocaine and heroin, and is another reason people keep smoking: to sustain high dopamine levels.

It has been noted that the majority of people diagnosed with schizophrenia smoke tobacco. Estimates for the number of schizophrenics that smoke range from 75% to 90%.
why do you think random people are having panic attacks? why do you think people's heart rates sometimes skyrocket? why do you think these people are suddenly not hungry, and instead want to vomit? why do you think weed has been coorelated with schizophrenia? its not the weed, its fucking tobacco in the weed.

no wonder its so hard to quit.