Pharmacodynamic interactions only. It seems kava deepens and lengthens the sedative nature of any GABAergic substance. This would make sense if constituents such as kavain are working as general allosteric modulators, causing the channel of the GABA-A receptor to remain open longer, or for more frequently when in the presence of endogenous ligands, as well as allosteric modulators such as BZP drugs.
Shinomiya, Kazuaki, Toshio Inoue, Yoshiaki Utsu, Shin Tokunaga, Takayoshi Masuoka, Asae Ohmori, and Chiaki Kamei. 2005. “Effects of Kava-Kava Extract on the Sleep-Wake Cycle in Sleep-Disturbed Rats.”
Psychopharmacology 180 (3): 564–69.
https://sci-hub.se/10.1007/s00213-005-2196-4
Tsutsui, Ryuki, Kazuaki Shinomiya, Yasuhiro Takeda, Yoshihito Obara, Yoshihisa Kitamura, and Chiaki Kamei. 2009. “Hypnotic and Sleep Quality–Enhancing Properties of Kavain in Sleep-Disturbed Rats.”
Journal of Pharmacological Sciences advpub: 0910300293–0910300293.
https://sci-hub.se/10.1254/jphs.09167FP