Benadryl, Zyrtec, and Other OTC medicine Wait Time Rules for Pilots

This episode originally ran on the Aviation News Talk podcast. In it, Max Trescott interviews Dr. John Trowbridge, a physician and former senior Aviation Medical Examiner, to tackle a hidden yet critical safety topic: how over-the-counter (OTC) and prescription medications contribute to general aviation accidents. Studies have found that up to 40% of fatal accidents involve pilots with impairing substances in their system—ranging from allergy medications to sleep aids to alcohol. The problem? Many of these substances are legal and even commonplace, yet can significantly degrade judgment, memory, attention, and coordination.

Dr. Trowbridge emphasizes that many pilots—and even their doctors—are unaware of FAA wait-time guidelines. He explains the FAA’s “5x rule,” which states that a pilot must wait five times the recommended dosage interval before flying. So if a medication is taken every six hours, the pilot should wait 30 hours after the last dose. For 24-hour medications like Zyrtec, the wait time stretches to five full days.

The discussion highlights the particular dangers of first-generation antihistamines like Benadryl (diphenhydramine), which are highly sedating and frequently found in sleep aids like Tylenol PM, NyQuil, and Unisom. These medications, even when taken the night before, can impair cognitive function well into the next day. Alarmingly, Benadryl is the most commonly detected OTC drug in fatal GA accidents.

Dr. Trowbridge also warns about second-generation antihistamines like Zyrtec and Xyzal. While marketed as “non-drowsy,” these can still cause subtle sedation, especially in combination with alcohol or other medications. Alternatives like Allegra and Claritin are usually safer and FAA-approved—but only after personal ground-testing and AME consultation.

Beyond antihistamines, they explore other drug categories. For pain relief, medications like aspirin, Tylenol, ibuprofen, and Aleve are generally safe, but anything with “PM” on the label likely contains sedating ingredients. Prescription painkillers like codeine are outright disqualifying. Dr. Trowbridge shares unconventional options too, like topical lidocaine, coconut oil, and even horse liniment—though with cautions about application and legality.

Sleep aids are another minefield. Melatonin is the only one on the FAA’s “go list,” and even it should be ground-tested first. Nasal decongestants such as Afrin and Sudafed can raise blood pressure and cause jitteriness, making natural remed