Nicotine
One of the most commonly used drugs is nicotine. Shortly after entering the bloodstream,
nicotine begins mimicking one of the body’s most common and important neurotransmitters:
acetylcholine. Fooled by the nicotine binding to acetylcholine receptors, cells release adrenaline
and other stimulating chemicals, including pleasure-causing dopamine. Nicotine causes rapid
surges, then rapid depletions, of these chemicals, leaving smokers happy for a short while but
soon yearning for another cigarette.
When we consume drugs such as those just described, our bodies soon respond to the
perception that there are larger amounts than usual of certain neurotransmitters. Usually, the
response is a reduction in sensitivity to the drug. Although tolerance is inevitable, its magnitude
and consequences vary with the type of drug. In one study, volunteers were injected with a
uniform daily dose of heroin and monitored for their level of euphoria. Initially, they were
ecstatic, but their bodies reacted by reducing the number of receptors that bind heroin. With
fewer and fewer receptors, the euphoric heroin effects dropped almost to zero in just three
weeks. Similar changes occur in response to using caffeine, nicotine, or alcohol.