If you believe some of what’s out there on the internet, hair drug tests can detect if you’ve ever take a drug anytime in your life.

Fortunately, most of what is out there on the internet is wrong.

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How Far Back Can a Hair Drug Test Go?

First off, drug tests work by looking for the byproducts your body creates as it metabolizes the drug in your body. These are called metabolites. So, if your hair is being tested for marijuana usage, it’s not testing for THC. It’s testing for THC-COOH which is what your body turns THC into.

However, not all drug metabolites are stable. THC-COOH is very prone to breaking down into other chemicals over time. Common environmental factors like exposure to UV radiation
(i.e the sun) or heat have been shown to cause THC metabolites to break down.

So if the test is looking for THC-COOH and the THC-COOH has broken down into other chemicals, drug tests that aren’t looking for these other chemicals (i.e. all commercially available drug tests) will not detect them.

This is why drug test labs only claim to be able to go back 90 days. They know that their tests become increasingly unreliable over time.

The way that drug testing companies limit their test to a 90-day window is by only testing 1.5 inches of your hair closest to the scalp. The remainder of the hair is discarded (for testing purposes).

how far back do hair drug tests go

When is 90 days not 90 days?

The reason that 1.5 inches of hair is considered to represent 90 days worth of hair growth is because that is the scientifically agreed upon average. For some people, the same 1.5 inches might be 100 days and for others it might be 80 days.

Certain medical conditions and nutritional factors can also influence the rate of growth.

There is also some evidence that there are racial differences in hair growth rates.

Another point to keep in mind is that drug metabolites are deposited into your hair while the hair is still growing beneath the surface of your scalp. It takes approximately 7 – 10 days for the hair to grow above the scalp line so if you quit taking drugs exactly 90 days ago, there could potentially be 7 – 10 days worth of drug metabolites still in the 1.5 inches of hair sampled.

Also complicating the window of detection problem is the fact that all of this only applies to hear grown on the head. If a bald man goes into a drug testing lab and a hair sample is taken from his underarms, there is no agreed upon rate of growth that would allow the drug testing facility to give an accurate exposure window.

In fact, Quest Diagnostics, one of the leading drug testing labs, admits this:

“The growth rate and rates of incorporation of drugs into hair from locations other than the head has not been studied as extensively as that of head hair. Consequently, one cannot reliably determine the window of detection of drugs using hair from alternative body sites.”(1)

Strangely, Quest Diagnostics routinely takes hair samples from places other than the head which would seem to call into question the entire test itself.

Off-Topic Note: It might be worthwhile consulting with a lawyer as to how this may factor into disputing a drug test result for a mandatory (i.e. court ordered) drug test.

So is it really 90 days?

For the most part, yes, but with some caveats. What you don’t need to be worried about is that the drug test can go back years, as some claim.

If you need to pass a hair follicle drug test your main concern should be on having a 90 day clean window prior to the test. Of course, the more wiggle room you can give yourself for variances in hair growth rates and such help but you should not concern yourself with any drug use considerably over that period.

(1) Quest Diagnostics Frequently Asked Questions Hair Testing

Pass a Hair Follicle Drug Test