Nicotine is a natural chemical produced primarily by tobacco plants as part of their defense mechanism against insects31. The plant synthesizes nicotine in its roots from two chemical compounds—pyridine and pyrrolidine—and then transports it to the leaves3.

In terms of what causes nicotine to be present in plants, the major factors include:

• Genetics: Tobacco and related plants in the nightshade family have specific genes that govern high-level nicotine production31.
• Environmental factors: Stresses like high temperature, flooding, and salt stress can increase nicotine production by activating certain plant signaling pathways (such as the jasmonic acid pathway)1.
• Hormonal regulation: Plant hormones, including jasmonic acid and auxin, regulate how much nicotine the plant produces, as does physical damage like "topping" (removing flowering tops)1.

For humans, nicotine dependence develops from repeated exposure to nicotine, primarily through tobacco product use24. The main causes and risk factors for nicotine dependence are:

• Genetics: Some individuals are more predisposed to addiction due to genetic variations affecting nicotine receptors in the brain2.
• Early use: Starting tobacco use at a young age increases the risk of becoming dependent2.
• Social environment: Family and friends’ use of tobacco can increase risk2.
• Co-existing conditions: Mental health problems and the use of other substances also raise the likelihood of nicotine dependence2.

When people use tobacco, nicotine is quickly absorbed into the bloodstream, travels to the brain, and binds to nicotinic acetylcholine receptors. This releases neurotransmitters, causing effects such as enhanced mood, increased arousal, and eventually, dependence and addiction4.

Summary Table: Causes of Nicotine (in plants and humans)

In summary, nicotine is primarily caused by specific plant genetics and environmental signals in tobacco, while dependence in humans is caused by repeated exposure and several genetic, social, and psychological factors1234.