Review Of How Long For A Hair Drug Test References. Web most hair drug tests can detect heavy or regular use of certain illicit substances over the past three months. Web the hair follicle drug test, when using the standard 1.5 inches of hair, can detect previous drug use up to 3 months.

You’ll also be okay if your cannabis use over the past 90 days has been very. However, older research notes that these test the oil in skin that transfers. A hair follicle drug test, or a hair drug test, is done to screen a person for illicit drug use.