90 days: Hair follicle tests can detect cannabis use up to 90 days based on a 1.5 inch follicle sample. If the follicle length is shorter, the detection period will likely be shorter.
Answered 5/21/2016
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Follicle testing: In “Pharmacokinetics of Cannabinoids” by McGilveray IJ it is stated “Hair tests are more likely to detect regular than occasional marijuana use. One study found that 85% of daily users tested positive for marijuana, versus 52% of occasional smokers (1-5 times per week). Marijuana regular use may be detected in hair for months. Single use might not be picked up in hair.
Answered 10/23/2019
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