Drug Half-Life – Why It Matters

by Dr. John W. Beasley, MD – Airmen Medical Examiner
Professor Emeritus and Clinical Professor,
Department of Family Medicine
University of Wisconsin – Madison

If you take a drug, how long does it last in your system? I have one pilot I certify who had to use occasional “hydrocodone” for episodes of acute back pain. Now, with good reason, the FAA will not allow people to perform the duties of a pilot while they are using narcotics. So, the question comes up: How long after one of these episodes is it before one can fly safely? Some years ago at a conference for Airmen Medical Examiner (AMEs), this question came up, and the answer was: “Usually about five half-lives.”

For those of you whose pharmacology is a bit rusty, a “half-life” is the time it takes to get rid of one-half of the drug from your body.

To use the example of hydrocodone, the half-life is about 4 hours (although there can be considerable variability). Thus, if you took a preparation of hydrocodone containing 10 mg, there would be 5 mg left at the end of 4 hours (neglecting time to absorb the drug, etc), 2.5 mg left after 8 hours, 1.25 mg left after 12 hours, 0.625 mg left after 16 hours, and roughly 0.312 mg left after 20 hours.

That said, there is a whole set of FAA rules pertaining to specific drugs. The FAA has different policies for different medications. Those friendly feds are, quite properly, concerned about the underlying condition requiring the use (e.g. the back pain), and whether it will affect piloting abilities. They are also concerned about habituation – which is the reason that occasional but not regular use is sometimes permitted. And finally, they are appropriately concerned about the effects of the drug itself.

I must admit that there are some policies that I have trouble understanding, such as those pertaining to “zolpidem” (Ambien) which although the half-life is only about 2.6 hours, pilot activities are prohibited if taking it more than twice a week and 24 hours after taking the drug.

Some issues that may surprise you: Took a “Benadryl” for some allergic problem? Well, the half-life of that drug is between 2 and 8 hours. Best then to wait 40 hours after the last dose to be safe. (By the way, Benadryl is NOT approved by the FAA for use while flying – “loratidine” and “fexofenadine” are). Using something else? You can just go to Google and type in “half-life” of whatever drug you are wondering about and you will probably find what you need.

Some cautions: The metabolism can vary a LOT between individuals and moreover many drugs have active metabolites (products of the breakdown process) that also have effects.

Most drugs are metabolized in this way – but not all. Take for example, “ethanol.” The pharmacology is different. The metabolism doesn’t work by half-life,” but rather a constant amount is metabolized each hour. It’s more like the gas in my Mooney’s tank.The usage is a bit under 10 gallons per hour until I become a glider. It’s not 10 GPH the first hour, 5 GPH the second and so forth. Dang!

So, for example, if you go to a party one night and consume five 5-ounce glasses of wine (by which time you need a designated driver), you would have consumed about 6 ounces of ethanol. With considerable variability we could expect you to metabolize about 1 ounce per hour, so you may be clear of alcohol in about 6 hours.

The actual blood alcohol content will depend on many factors including body weight, rate of consumption, and gender. But that’s not all the story – the after-effects of alcohol also impair performance, perhaps up to twice the time it takes to reach zero blood alcohol. The 8-hour “bottle to throttle” rule may not be conservative enough. You could still technically be “under the influence” as far as aviation activities go. Of course, you’ve never had a hangover. Me neither.

What’s the take-home? If you do take any medications that could have adverse effects, keep the 5-half-life rule in mind. You can go to http://www.aopa.org/members/databases/medical/druglist.cfm to get a list of drugs that are allowed or not allowed. If in doubt, talk with your AME. It’s not possible here to list all the FAA policies – and many of them I would have to call the FAA myself to check.

August/September 2010     Midwest Flyer Magazine

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