This narcotic analgesic has no medical use and can be injected, snorted, or smoked. It replaces natural endorphins in the CNS. This drug is the most widely abused opioid and is:

A. heroin: Heroin is an illegal opioid with no accepted medical use in most countries. It acts on opioid receptors and replaces endorphins, leading to intense euphoria and high addiction potential. B. laudanum: A historic opioid tincture used in the 19th century for pain and diarrhea; rarely abused today. C. morphine: A potent opioid used medically for pain management. Abuse is possible but it's not the most commonly abused opioid. D. codeine: A milder opioid used medically in cough syrups and for pain. Less commonly abused than heroin.