Which drug always requires a prescription, regardless of the amount in a compound?

The correct answer is based on the legal classification of the drugs in question, particularly focusing on their potential for abuse and regulation under the Controlled Substances Act.

Phenobarbital is classified as a Schedule IV controlled substance, which means that it has accepted medical uses but also carries the risk of abuse and dependence. Because of its classification, any amount of phenobarbital must be dispensed with a prescription. This is a requirement that is enforced to monitor and control potential misuse.

Other substances listed in the options, while also regulated, have specific conditions under which they may not always require a prescription. For example, codeine is sometimes found in combination products that may not require a prescription depending on the dosage and formulation; hydrocodone is similarly regulated but can also appear in specific combination medications that might be available without a prescription in certain jurisdictions. Diazepam, a benzodiazepine like phenobarbital, also falls under strict scheduling but may also allow certain variations in access.

In summary, the distinction comes down to the nature of Phenobarbital's scheduling and the overarching legal framework requiring a prescription regardless of quantity when it comes to this compound, ensuring its regulated use due to its potential for abuse.