A hair test is designed to detect the presence of marijuana and other narcotics in the hair follicle.  Hair testing requires 1-1/2 inches of hair, representing about 3 months of hair growth and drug use.  Once a drug like cannabis is used, it takes about 7-10 days for the hair containing the drug to grow out of the scalp enough to be cut. Therefore, the hair test will not include drugs used in the week prior to the test.

Hair tests are more complicated than urine and saliva tests because they must be sent to a laboratory for analysis by a professional technician.  Hair tests are more expensive than both saliva and urine tests and are not usually required as part of a pre-employment screening process. Due to the cost of processing a hair sample, these test are used sparingly.

If you have a hair drug test coming up and have used marijuana there is a way to pass a hair test. These products will strip the outside hair of all drug use.