Abstract The effects of repeated hair washing and a single hair dyeing on concentrations of methamphetamine (MA) and amphetamine (AM) in hair samples of MA addicts were studied. Thirty-one MA positive hair samples collected from male ( n = 24, 24–51 yrs) and female abusers ( n = 7, 17–46 yrs) were evaluated for MA and AM concentration's changes after repeated hair washing and a single hair dyeing. Thirty-one MA positive hair samples, no additional treatment hair sample group (NAT), were treated in vitro with liquid soap or three kinds of hair dyes which were black, brown and yellow color hair dye, respectively. Quantitation of AM and MA in hair samples was utilized GC–MS using selected ion monitoring. MA and AM concentrations in NAT were 10.41 ± 8.91 ng/mg (range 1.50–30.0 ng/mg) and 2.24 ± 2.75 ng/mg (range 0.41–12.90 ng/mg). And, their concentrations were decreased about 23.3 ± 4.5% (range 16.7–32.8%) in hair repeated washing group (WAS) and 32.6 ± 4.82 (22.2–41.9) in three kinds of a single hair dyeing groups in comparison to original concentrations of MA and AM in NAT. A statistically significant difference was found between NAT and WAS or three hair dyeing groups ( p < 0.01), but not between WAS and three hair dyeing groups, and not between each hair dyeing groups with each three kinds of hair dyes ( p > 0.05).