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How far back does a hair follicle test detect drugs?

Medically reviewed by Sally Chao, MD. Last updated on Sep 27, 2023.

Official answer


Most hair drug tests can detect heavy or regular use of certain illicit substances over the past three months.

Your hair grows at a rate of about one-half inch per month from the scalp. The standard 1.5-inch sample taken near the root provides a snapshot of drug use over the past 90 days.

When the hair sample is taken from slower-growing body hair (such as from the armpit) instead of the scalp, the detection window widens to up to one year.

Traces of illicit drug use may be seen in a hair sample within one week of drug use.

Hair follicle drug tests

This type of drug testing can be used to identify:

Hair testing does not test for alcohol use.

Unlike other drug tests, hair follicle drug tests are not recommended for testing occasional use. Instead, hair drug testing can provide a rough timeline of drug use over an extended period. These tests can help detect chronic substance use and help track the duration of drug use, and they can also indicate periods of abstinence.

Hair sample interpretation may be affected by:

  • Sweat
  • Bleaching or coloring hair
  • Secondhand smoke

The hair sample is usually taken from the scalp as close to the back of the head as possible. Hair can also be taken from the armpit, face or other unshaven part of the body if it is unavailable on the scalp.

Related Questions

  1. Hadland SE, Levy S. Objective Testing – Urine and Other Tests. Child Adolesc Psychiatr Clin N Am. 2016; 26(3): 549–565.

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