Which of the following is an example of a controlled substance?

The correct answer is narcotics, which are classified as controlled substances due to their potential for abuse and dependency. Controlled substances are regulated under federal and state laws to prevent misuse. Narcotics, such as opioids and certain pain relievers, are included on schedules that outline their medical uses, potential for abuse, and safety profile.

In contrast, vitamin supplements and allergy medications generally do not have abuse potential and are not regulated the same way as narcotics. While antibiotics are prescription medications used to treat infections, they also lack the abuse potential that classifies narcotics as controlled substances. Thus, narcotics stand out as the primary example of a controlled substance.