Heroin is an illegal opioid drug made from morphine, a natural substance extracted from the seed pods of certain varieties of poppy plants.(Source) It typically appears as a white or brown powder or as a sticky black substance known as black tar heroin.(Source) People use heroin by injecting, smoking, or snorting it. Any route of use can be dangerous and addictive.

Once heroin enters the bloodstream, it quickly crosses the blood–brain barrier and is converted back into morphine, which then binds to opioid receptors in the brain and throughout the body.(Source) These receptors are involved in pain perception, reward, and breathing. When heroin activates these receptors, it triggers a surge of dopamine, a neurotransmitter linked to pleasure and reinforcement.(Source)

This intense rush of euphoria and relief from emotional or physical pain is one of the main reasons heroin is so addictive. Over time, the brain adapts to the presence of heroin, reducing its natural production of dopamine and other neurotransmitters. This leads to tolerance (needing more of the drug to feel the same effects) and physical dependence (experiencing withdrawal symptoms when not using). These brain changes are key features of opioid use disorder, the medical term for heroin addiction.(Source)