Cocaine, methamphetamine, opiates and phencyclidine (PCP) are quickly excreted from the body, and are usually undetectable within urine samples 72 hours after use. The detection period for hair is approximately 90 days for a standard test.
There are currently no known adulterants for hair tests. Because hair tests analyze the drugs inside the hairshaft, external contaminants/chemicals have absolutely no effect. The non-intrusive collection procedure basically eliminates the ability to avoid or delay the drug test.
The combination of being able to detect drugs for a greater period of time, and the elimination of the ability to avoid or delay the drug test makes hair testing far more effective and accurate than urinalysis in correctly identifying drug users.