Nicotine is a highly addictive chemical found in tobacco products. When nicotine is absorbed into the bloodstream, it makes its way to the brain and causes the release of dopamine, a chemical that produces feelings of pleasure and reward. This "rush" is a major part of the addictive process. Over time, the brain begins to crave that feeling from nicotine and people need to use more and more tobacco to get that same good feeling. Nicotine also alters the sensitivity of the brains reward system, leading to changes in other brain circuits involved in learning, stress, and self-control. Long-term changes in the brain caused by continued nicotine exposure result in addiction, which involves withdrawal symptoms when not smoking and difficulty adhering to the resolution to quit. Nicotine dependence involves physical and psychological factors that make it difficult to stop using tobacco, even if the person wants to quit.