Which opioid is often used as a cough suppressant in subanalgesic doses?

The correct choice is codeine, as it possesses antitussive (cough suppressant) properties at subanalgesic doses. Unlike many other opioids, which primarily function to relieve pain, codeine specifically acts on the cough center in the brain to help suppress the urge to cough. This makes it particularly useful in managing coughs that may not respond to other treatments, especially when used in lower doses that do not produce significant analgesic (pain-relieving) effects.

Morphine is primarily known for its strong analgesic properties and is not typically used for cough suppression. Oxycodone is also an effective analgesic and does not have the same established use as a cough suppressant. Percocet is a combination medication that includes oxycodone and acetaminophen, primarily prescribed for pain relief without cough-suppressing indications. Thus, codeine stands out as the opioid that is specifically designated for use in alleviating cough symptoms, making it the appropriate answer in this context.