AGAZINE - Midwest Flyer

Hi g h On He a l t h
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
Dr. John Beasley
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 “halflife”
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
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16 AUGUST/SEPTEMBER 2010 MIDWEST FLYER MAGAZINE
drug, etc), 1.5 mg left after 8 hours,
0.75 mg left after 12 hours, 0.375 mg
left after 16 hours, and roughly 0.16mg
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
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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.
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