Which of the following antitussives is classified as a scheduled substance?

Codeine is classified as a scheduled substance due to its potential for abuse and dependence. It is an opioid that acts on the central nervous system to suppress cough reflexes. In many countries, including the United States, codeine is regulated as a Schedule II or Schedule III controlled substance, depending on its formulation and concentration. This classification means that when used as a cough suppressant in certain formulations, it is available by prescription only and is monitored closely to prevent misuse.

The other substances listed do not have the same level of regulation. Dextromethorphan, while commonly used as a cough suppressant, is not a controlled substance. Guafenesin is an expectorant and has no abuse potential, thus it is not regulated. Diphenhydramine is an antihistamine that can be used for cough but is also not considered a controlled substance. Therefore, the classification of codeine as a scheduled substance highlights the importance of monitoring its use due to its associated risks.