Hair needs blood to grow and hair follicle sources blood to hair. The moment you are consuming drugs, traces of drugs, blood start carrying and it deposits onto the hair shaft by means of hair follicle. Either the drug itself gets shifted to the hair follicle or the residue of the metabolites that the body generates in company of drugs are seen to be deposited onto the hair follicle. The moment you are allowing your hair to grow, the chemicals which are deposited along the hair follicle, develop with the drug keeping the record of it. It can be contrasted with a tree which grows with the rings testifying the effect of changing season. When it is about testing through the hair follicle, it is seen that little sample of your hair is sliced from the back of your head, slicing it along the skin and kept in within the sealed container. The sample never goes more than 1.5 inches and for 90 days the hair sample can be kept. Next the sample is sent for lab test only to trace chemicals.
Now there are number of drugs which can be detected with hair follicle drug test Alcohol, Marijuana, Cocaine, Amphetamines, Opiates, Barbiturates, PCP, Nicotine, LSD. There are ways to cheat this drug test such as shaving off the hair , then applying specialized DIY kits to pass a hair follicle drug test for example, using detox shampoos, products and solutions to wash off the chemicals then there are some detoxifying products which are best in clearing out the drugs.