Hair Detection "Window"
"An advantage of drug testing with hair is the longer window of detection compared with other matrices (Boumba, Ziavrou, & Vougiouklakis, 2006). The detection period for hair is limited only by the length of the hair sample and the degree of deposition in the hair. Cannabinoids have been shown to deposit less readily than basic drugs in hair (Huestis et al., 2007). Some laboratories typically restrict analysis to a hair segment representing about 3 months of growth. However, this long window period is also a disadvantage; hair testing is not useful in substance abuse treatment or monitoring opioid pain or other addictive medications when frequent (weekly or monthly) drug testing is desired. Because the timing of the drug use is difficult to determine by testing hair, it is not very useful clinically."
"Clinical Drug Testing in Primary Care," Technical Assistance Publication (TAP) 32, Substance Abuse and Mental Health Services Administration, Clinical Drug Testing in Primary Care (Rockville, MD: U.S. Department of Health and Human Services, 2012), p. 22.