I'm not sure of that. If you don't have long enough hair on your head due to shaving or male pattern baldness they will get hair from another part of your body. That's if you don't shave your whole body.
I worked for a company that made telecommunication equipment and one day this guy comes into work with his head shaved. I later found out that he was going in for a hair drug test. Never saw the guy again and heard that they couldn't get a hair sample from anywhere on his body. The reason for the rest was you are hired on through a temp agency then after six months they will hire you pending a hair test. Supposedly they can go back six months with hair test. Apparently drugs pass through your system quicker where you can't get an accurate six month reading through a piss test.