However, cosmetic procedures can change drug concentrations in hair samples, resulting in misleading negative hair test findings. Oxidative bleaching of hair samples under alkaline circumstances, in particular, has a considerable effect on integrated drug concentrations. The degree of reduction depends on the type of substance and length of treatment. Hair coloring products contain substances that can enter the bloodstream through broken skin or even by swallowing contaminated powder. This can lead to positive drug tests after just one use of colored hair. Professional hair dyeing services should be performed by trained professionals to avoid exposure to toxic chemicals. Bleached hair will not pass drug tests because it no longer contains the black pigment that would show up on laboratory analysis. However, the oxidized chemicals present in bleached hair may still trigger a positive result from another source. It is important to know that certain medications can cause false positives for THC (the main psychoactive component of marijuana). These drugs include some anti-anxiety medications, painkillers, heart medications, antidepressants, and diabetes medications. If you think this might be the case for you, discuss medication with your doctor before going for hair color therapy. He or she can suggest alternatives that won't cause false positives. Hair bleach can also affect the outcome of a urine drug test. Laboratory analyses usually include a pH test to determine whether the sample comes from dyed or naturally dark hair.