•A LC-ESI-MS/MS method was developed for the quantification of kavain in mice plasma.•This simple and fast method was fully validated according to regulatory guidelines.•Isotopically labeled kavain was used as internal standard for the developed method.•Pharmacokinetic profiles of kavain isolated and in kava-kava extract were compared.•Higher bioavailability for kavain present in the extract was observed due to pharmacokinetic synergism. A simple and fast bioanalytical method for the quantification of kavain in mice plasma was developed using liquid chromatography (LC)-tandem mass spectrometry (MS/MS). A full method validation was performed, according to regulatory guidelines, employing isotopically labeled kavain as the internal standard (racemic-kavain-d3). For the quantification, M+H+ was formed using an electrospray ionization (ESI) source in the positive ion mode and multiple reaction monitoring (MRM) was employed using a quadrupole-linear ion trap (4000 QTRAP®) instrument. The monitored MRM transitions were 231.0 → 115.1 and 231.0 → 152.8 for kavain; and 234.2 → 199.2 for the internal standard. A linear response was obtained at the concentration range of 10 to 200 ng/mL with intra- and inter-day variations within the acceptable criteria for all quality control samples. After validation, the method was successfully applied for the quantification of kavain in mice plasma after oral administration of the kavain standard and Kava-kava extract. The plasma concentration over time results were applied for a pharmacokinetics study. The obtained pharmacokinetic parameters indicated a considerably higher bioavailability for kavain when Kava-kava extract was administered due to a pharmacokinetic synergism between the analyte and the other compounds present in the extract.