Which of the following substances is classified as an opioid?

Hydrocodone is correctly classified as an opioid because it belongs to a class of drugs that act on the opioid receptors in the brain, which are part of the body's pain control system. Opioids are primarily used for pain relief, but they can also create a sense of euphoria, which can lead to their misuse. Hydrocodone is often prescribed to manage moderate to severe pain and is commonly combined with acetaminophen or ibuprofen in various formulations.

In contrast, the other substances listed do not belong to the opioid category. Ketamine is primarily used as an anesthetic and has dissociative properties, affecting glutamate neurotransmission rather than binding to opioid receptors. Cocaine is a powerful stimulant derived from the coca plant and acts on dopamine pathways in the brain, not on opioid receptors. Alprazolam is a benzodiazepine that is used for anxiety and panic disorders; it works on the GABAergic system, which is different from the opioid mechanism of action.