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.
Plain English
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.
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 nitrogen-based compound). 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.
Why Pilots Care
The FAA prohibits flying while using diphenhydramine because drowsiness and slowed reaction time increase the risk of errors and accidents.
Intuition Check
Do not assume “over-the-counter” means “safe to fly.” Diphenhydramine is available without a prescription, but it can still impair a pilot.
Example Sentence 1
She took diphenhydramine for her allergies the night before, so she scrubbed the morning flight to stay within the FAA's recommended waiting period.
Example Sentence 2
Any medication with diphenhydramine must be avoided before a flight because it causes drowsiness that can last several hours.