Heroin rapidly crosses the blood-brain barrier and converts to morphine, binding to opioid receptors. This triggers intense euphoria by flooding the brain with dopamine, reinforcing reward pathways. It also suppresses pain and slows breathing. Repeated use alters brain chemistry, reducing natural endorphin production and weakening self-control circuits. Over time, tolerance, dependence, and addiction develop, with high risk of overdose due to respiratory depression and impaired judgment.