Addictive substances affect the brain and nervous system by interfering with neurotransmitters, resulting in negative effects such as: Mood swings. Unpredictable behavior. Different interactions with others. This can lead others to perceive the user as abnormal and avoid them. The effects of drugs on the nervous system extend to most brain regions that control: Logical reasoning. Remembering events. Controlling behavior and emotions. Judgment. Therefore, brain functions in users of these substances are negatively impacted, leading to mental and behavioral impairment. Al-Mustaqbal University, the leading university in Iraq.