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How Does a Drug Hair Test Work?

  • By: Andrew Easler, Esq.
  • Published: Oct, 6 2014
  • Updated: Dec, 20 2022

Hair follicle testing can be done to determine the presence of a variety of illicit drugs, including marijuana, cocaine, methamphetamine, ecstasy, phencyclidine (PCP), and opiates like morphine. It can also be used to determine the presence of date rape drugs or alcohol. Results are typically accurate up to 90 days, and the ease of observing the collection process means that there is less chance of adulterating the sample than there would be with urine testing. For that matter, there are no ways of tampering with a hair test – the hair collector takes the sample, and it’s pretty much determinable that it belongs to the subject being tested.

How Does Hair Testing Work?

About an inch and a half of hair is used. This generally represents approximately three months’ growth, since hair usually grows at a rate of about half an inch per month. The generally accepted rule is that in order for the test to come out positive, the drug that’s being tested for has to have been used at least three times. After the drug has been used, the hair nearest the scalp is drug-free. The drug “grows out” along with the hair. The closer the drug is to the scalp, the more recent the drug use.

Do Medications or Other Ingested Substances Result in False Positives?

It’s remotely possible. Some prescriptions contain the same compounds as “street drugs,” and there’s no way to distinguish between the two. However, if a person has been using a drug for medical purposes, his or her medical record will reflect that use.

Some prescription diet pills contain amphetamines, and so does a common drug that’s prescribed for Parkinson’s Disease. Again, this will be revealed on the medical record.

There is one big exception, though – you’ve heard that poppy seeds can result in testing positive for heroin? It’s actually true. Eating just one poppy seed bagel can result in a positive drug test.

What About Second-hand Drugs?

If a person is in a room with a person who is using drugs, it’s unlikely that it will result in a positive drug test. It takes multiple uses in order to test positive, so passive exposure isn’t likely to deliver a positive test, unless the exposure is extreme, in which case the level of the drug present will still be extremely low, and therefore easily identified as a false positive.

Will Hair Dye Result in a Negative Test?

No. The drugs aren’t on the hair, they’re IN it. Drugs travel up into the hair shaft from the bloodstream. So dyeing and bleaching, and even washing, will lower any drugs that have become deposited on the hair (again, perhaps from second-hand exposure), but it will not remove the drugs that are actually in the hair shaft. Before drug testing, in any case, the hair collector will wash the test subject’s hair to make sure that any drugs that show up will be from the hair shaft.


Intriguing, isn’t it? Hair follicle testing can be a fascinating career, and courses are easily obtainable that will help you get started.

The information on this page may have changed since we first published it. We give great legal advice, but this page (and the rest of our site) is for informational use only and is no substitute for actual legal advice. If you’d like to establish an attorney-client relationship, reach out to us and we’ll tell you how we can make it official. Sending us an email or reading this page alone doesn’t mean we represent you.

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