Which of the following active ingredients is typically regulated under controlled substances?

Buprenorphine is an active ingredient that is regulated under controlled substances due to its classification as an opioid. It is used in the treatment of pain and in medication-assisted treatment for opioid use disorder. The regulation of buprenorphine stems from its potential for abuse and dependence; it acts on the same receptors in the brain as other opioids, though it has a "ceiling effect" that makes it less likely to be abused compared to stronger opioids like morphine or fentanyl.

Other options, such as ibuprofen, diphenhydramine, and acetaminophen, are not classified as controlled substances. Ibuprofen and acetaminophen are commonly used analgesics that can be purchased over-the-counter without special regulations. Diphenhydramine is an antihistamine often used for allergies and sleep aids, also available without prescription and not subject to control. Therefore, the regulation of buprenorphine highlights the concerns around safety, misuse, and dependence associated with certain medications, particularly those that can affect the central nervous system.