Nicotine is a chemical that acts as the addictive agent in cigarettes. It is a psychoactive stimulant that induces feelings of relaxation and reduced stress. Nicotine withdrawal causes intense irritability, restlessness, and difficulty concentrating. The risk of nicotine addiction depends on the dose and method in which nicotine is delivered to the brain.
Nicotine, especially when ingested or through skin exposure, can be harmful and even lead to death at amounts around 50-60 mg of nicotine. Exposure to liquid nicotine was relatively rare until the introduction of e-cigarettes.