nicotine

n. The psychoactive chemical in tobacco, which works by mimicking acetylcholine and triggering the release of dopamine, norepinephrine, and vasopressin. In low doses nicotine produces a stimulating effect with increased alertness and calmness while at higher doses it causes dizziness, diarrhea, vomiting, tremors, and paralysis and can cause death by paralyzing the diaphragm. In its common usage patterns nicotine is the most highly addictive drug.