Heroin is an illegal, highly addictive opioid prepared from morphine. Typically injected, heroin can also be smoked or snorted. Heroin is especially dangerous because it is often mixed with other drugs, including fentanyl, which increases the risk of overdose. In addition, when people inject heroin, they risk contracting serious, long-term viral infections, such as HIV, hepatitis C, and hepatitis B, as well as bacterial infections of the skin, bloodstream, and heart. When people overdose on heroin, their breathing often slows or stops. This decreases the amount of oxygen that reaches the brain and can cause coma, death, or permanent brain damage.