Plain English Meaning

A common over-the-counter allergy and sleep medicine (most familiar as Benadryl) that makes people drowsy and slow to react, so pilots are not allowed to fly while it is still affecting them.

Full Definition

A first-generation antihistamine medication used to treat allergy symptoms, motion sickness, and as a sleep aid. It commonly causes drowsiness, slowed reaction time, and impaired judgment, and is on the FAA's list of medications that are disqualifying for flight while their effects remain in the body. The drug is best known by the brand name Benadryl.

Context Anchor

Pilots encounter this term when checking whether an over-the-counter allergy, cold, or sleep-aid medicine is safe before flying.

Derivation

The name comes from its chemical structure: di- (two) + phenyl (a chemical ring group) + hydr- (hydrogen) + amine (a -based ). Knowing the parts isn't important for pilots — what matters is recognising the name on a medicine label so you can spot it as a no-fly drug.