389 Benadryl, Zyrtec, and More: FAA Wait Time Rules for Pilots with Dr. John Trowbridge

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 remedies like saline rinses or cool vapor inhalation preferable.

Cough medications also pose risks. Products with dextromethorphan (like DayQuil or Delsym) can sedate, as can multi-symptom formulas marked “PM” or “nighttime.” Gastrointestinal issues are more straightforward: most antacids like Tums and Maalox are safe, but anti-diarrheals like Imodium are not, due to sedation risks. UTIs are covered with non-sedating options like AZO and D-Mannose, but Dr. Trowbridge cautions pilots never to fly if symptomatic or on unfamiliar antibiotics.

The conversation then turns to alcohol. The FAA’s limit is 0.04%, but even lower levels can impair judgment, night vision, and reaction time—especially when combined with other medications or altitude-related hypoxia. Max cites an older FAA study showing that alcohol above 0.04% was found in 7% of fatal pilot crashes, with 3% involving both alcohol and drugs.

Finally, Dr. Trowbridge emphasizes the importance of pilot self-awareness and due diligence. Most doctor