Which of the following drugs is not allowed for a driver without a prescription?

Amphetamines are classified as controlled substances and are not permitted for use by drivers without a prescription. This is due to their potential for abuse and significant effects on a person's ability to operate a vehicle safely. The FMCSA regulations emphasize the importance of maintaining safety in transportation, and allowing amphetamines without medical oversight poses a serious risk to public safety.

In contrast, the other substances mentioned—Ibuprofen, Acetaminophen, and Aspirin—are over-the-counter medications that are generally considered safe for most people to use without a prescription. They do not impair a driver's ability to perform job functions when taken as directed, making them permissible for drivers under FMCSA guidelines.