In the past, professionals assumed gateway drug function was caused by social and psychological factors. Recent discoveries in brain neurophysiology suggest a strong pharmacologic component to nicotine’s gateway drug function. Addiction researchers note that tobacco use increases the level of the neurotransmitter dopamine in the brain. Drug induced higher levels of dopamine partially explain why psychoactive drugs such as nicotine, alcohol, cocaine, or heroin are addictive.

Serendipitous findings by researchers at the Brookhaven National Laboratory provided an additional neuropharmacologic explanation for nicotine addiction as well as its gateway drug function. Researchers discovered that tobacco use causes a reduction in the enzyme monoamine oxidase B or MAO-B which is responsible for the breakdown of dopamine. Smokers had 40% less MAO-B than nonsmokers or former smokers. Reduction in this critical enzyme may explain why nicotine addiction is so difficult to break. Smoking creates a cycle where reduction in MAO-B causes more dopamine, which causes greater pleasure for smoking, which leads to more smoking, which causes less MAO-B. Smoking-induced reduction of MAO-B appears to cause a synergist effect with the dopamine-stimulating effect of smoking by slowing the breakdown of this pleasure-enhancing neurotransmitter.

By the same mechanism, smoking may enhance the pleasure that results from using heroin, cocaine, alcohol, and other psychoactive drugs. According to the Brookhaven researchers, if tobacco use can increase dopamine levels in the brain by inhibiting MAO-B, “it would give a neuropharmalogic basis to the proposal that cigarettes are a ‘gateway drug’.” according to an article in the March 11. 1996 issue of Time on “How smokers get hooked.” These researchers believe nicotine may create a biochemical pathway or channel so that the next drug becomes more pleasurable than it would otherwise.” Thus, an adolescent who regularly uses tobacco will undergo brain chemistry adaptations. These brain chemistry changes heighten the pleasurable effects of taking other drugs, and the reinforcement increases the likelihood that the adolescent will become addicted to other substances. As evidence that tobacco use facilitates development of other addictions, up to 90% of individuals in drug rehabilitation programs for alcohol or illicit drugs concurrently use and are addicted to nicotine. Smoking rates for the general population approach 25%. The pharmacologic ability of tobacco to facilitate development of dependence to other drugs may partially explain why nicotine use nearly always occurs in people addicted to other drugs.