For a hair sample drug test how far back do they usually look?

The analysis of hair for drugs of abuse is predicated on the fact that drugs that are ingested are circulated in the blood, which in turn nourishes the hair follicle. Small amounts of the circulating drugs are deposited in the hair shaft. These deposits are roughly proportional to the amount of drug originally ingested by the individual. These deposits of drug remain in the hair as it grows at the rate of approximately ½ inch per month. These drug deposits are not washed or flushed out of the hair and do not diminish over time, but are released only upon the destruction of the hair fiber. Drug use can thus be detected months or even years following ingestion, depending upon the length of the sample collected.

1 year

"Hair follical" tests are the most common and test for recent drug use. Tests on hair itself go back further. Hair grows an inch or so in 30 days, so the hair from a year ago is a foot or so from the scalp. I also believe that washing or treating hair lessens the presence of the chemicals used to identify drugs use so hair further from the scalp (which has been washed or treated more) is less effective in indicating drug use. There are treatment you can buy that say they can make hair tests ineffective. I don't know if they really work though. See here http://www.cleartest.com/

They test the whole hair, so how far back they can check depends on the length of the hair

The whole growth of the hair. Depends on how long your hair is. Some drugs will show up one year later.

I am not sure how true this is but I was once told that hair samples can go back as far as 5 years. Not really sure so don't quote me on that, it's just what I have heard.

well not to the scalp,and the drugs are "IN" the frolics of the hair, so if you did it in the last 6 months, it will show

6 months is the limit

well if you have real long hair they can check way back the shorter the hair the shorter they can go back ..p.s they usually use pubic hair..if available

they try to expose the roots of your drug problem