Final answer:
Robitussin AC requires a prescription because it contains the opioid codeine, which is a controlled substance.
Step-by-step explanation:
Yes, Robitussin AC does require a prescription. Unlike other types of Robitussin that are available over the counter, Robitussin AC contains codeine, which is an opioid, and guaifenesin. Due to the presence of codeine, which can be addictive and is a controlled substance, Robitussin AC can only be obtained with a doctor's prescription to ensure safe and monitored use.