Patchouli essential oil (PEO) was provided by Bio-Jourdeness International Groups Co., Ltd. (Taichung, Taiwan). Patchouli alcohol was obtained from Biosynth International, Inc. (San Diego, CA, USA). The compound's purity was determined to be above 99%, as confirmed by both gas chromatography (GC) and proton nuclear magnetic resonance ( 1 H-NMR) analyses. Fetal bovine serum (FBS), Roswell park memorial institute (RPMI) 1640 medium, penicillin, and streptomycin were procured from Life Technologies (Grand Island, NY, USA). 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT), tyrosinase (EC 1.14.18.1, activity of 6680 units/mg), melanin, and kojic acid (KA) were purchased from Sigma-Aldrich (St. Louis, CA, USA). Forskolin (FRK) was acquired from Selleckchem (Houston, TX, USA). An antibody against tyrosinase was obtained from Genetex, Irvin, CA, USA. Antibodies against GAPDH, tyrosinase-related protein-1, and tyrosinase-related protein-2 were obtained from Santa-Cruz Biotechnology (Dallas, TX, USA). Horseradish peroxidase (HRP)-linked anti-mouse IgG and anti-rabbit IgG antibodies were sourced from Cell Signaling Technology (Danvers, MA, USA). All other chemicals used were of reagent grade or HPLC grade and were provided by either Merck (Darmstadt, Germany) or Sigma-Aldrich.