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Benadryl and Flying: A Deadly Cocktail

What level of cognitive impairment would you like to have when performing piloting duties? If your answer is "none at all", then it is imperative to be aware of what medicines you take before flying, especially if they possibly contain Benadryl®. As an aerospace medicine doctor and researcher, I cannot tell you the number of fatal accident cases I have reviewed that involve the discovery of Benadryl® in the deceased pilot's blood. This is not just my observation. In a study performed by the FAA ( looking at drugs and alcohol in fatal civil aviation accidents during the 2004 to 2008 period, diphenhyhdramine (brand name Bendaryl®) was the most common drug found in pilots who had died.

Diphenhydramine is a sedating antihistamine drug that is commonly found in many over-the-counter (OTC) cold, allergy, and sleep aid medicines. Medlineplus lists a total of 51 OTC medications that include diphenhydramine. With cold and flu season upon us and allergy season around the corner, it is more than worth it to review the list and make sure any medicines you may be taking immediately prior to flying do not contain diphenhydramine. Doxylamine is another sedating antihistamine incorporated in combination cold, flu, and allergy medicines that should be avoided immediately prior to flying. The physical impairments and precautions are the same for both drugs. You can check the Medlineplus list of medicines for diphenhydramine at and for doxylamine at

One of the main locations in your body that can be negatively affected by Benadryl® is your vestibular system (inner ear) which is responsible for establishing your spatial orientation and sense of balance. It is the main system that is affected when you spin around in circles and become dizzy and it is significantly affected by Benadryl®. Because of this, Benadryl's® cognitive impairing side effects profile include:



Impaired coordination

Low blood pressure

Double vision

among other side effects, almost all of which are undesirable in the cockpit. What's more, Benadryl® may interact with other sedating medicines to increase the negative side effects thus magnifying impairment.

If Benadryl® is the only medicine you find that can provide relief for your cold, allergy, or sleep symptoms and you feel you cannot give it up, you should wait at least five (5) half-lives before getting in the cockpit. Benadryl's® half-life is quite long, up to 10 hours. This means waiting at least 50 hours before operating your aircraft. Doxylamine's half-life can be as high as 15 hours (75 hours of wait time before flying). If this is too long to wait, you should not exercise your piloting duties, have someone else pilot the aircraft, or cancel your flight. In any case, YOU SHOULD NOT FLY!

Several factors might slow down how Benadryl® is processed in your body: the use of other medications in combination with Benadryl® and/or liver or kidney problems. Benadryl®, like many other medications, is processed by the liver and kidneys. If your liver or kidneys are impaired for any reason, this very well could slow down processing and increase the time Benadryl® stays in your system. Equally, it is possible to slow down the processing of Benadryl® if your liver and kidneys are working hard to process alcohol or other medicines. Much like a highway overloaded with traffic, the liver and kidneys can be overwhelmed by medicines, slowing down the processing of Benadryl®.

In both cases, you would have to wait longer before flying in order to be safe. If you take other medications along with Benadryl®, or have liver of kidney dysfunction, you should speak to both your doctor and your pharmacist regarding Benadryl's half-life and when it would be cleared from your system. In the FAA's review of civil aviation accidents, cited above, they warn that the manufacturer's listed half-life may not be adequate, that it could actually be a longer half-life. The National Transportation Safety Board (NTSB) has produced a flyer for pilots regarding general medication use and precautions (

The bottom line is "beware of what medicines you are taking and how they can effect your flying performance." Review your medication side effects - ask your AME - ask your doctor - ask your pharmacist. What ever you do, do not just ignore them. It could mean your life! If nothing else, I hope this article gets you thinking about the effects of Benadryl® and possibly saves at least one pilot's life. Please leave comments or questions below and/or share your experiences. Lets share the knowledge - safety first.

If you have a medical topic to discuss or a medical question you would like to have answered, please send an email to or leave a comment with your question(s).

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