Kava has the ability, similar to grapefruit juice, to suppress certain enzyme pathways in liver. Specifically the enzyme CYP1A2, the liver enzyme responsible for the majority of caffeine's metabolism. What this means is that kava can and does interfere with compounds broken down by the same pathways such as caffeine. Caffeine is prevented from being ""digested"" into its constituent parts, therefore causing a lengthening and strengthening of its effects until it finally is broken down. According to the following study kava may decrease the rate of metabolism by up to 56% which isn't negligible. The effects could even further increase or be altered as you go down the metabolite tree, as kava has inhibition properties at many different commonly seen pathways.
""In the present work the inhibition of P450 enzymes by kava extract and individual kavalactones in human liver microsomes (HLMs) was investigated. Whole kava extract (normalized to 100 microM total kavalactones) caused concentration-dependent decreases in P450 activities, with significant inhibition of the activities of CYP1A2 (56% inhibition), 2C9 (92%), 2C19 (86%), 2D6 (73%), 3A4 (78%), and 4A9/11 (65%) following preincubation for 15 min with HLMs and NADPH; CYP2A6, 2C8, and 2E1 activities were unaffected. The activities of CYP2C9, 2C19, 2D6, and 3A4 were also measured after incubation of HLMs with the major kavalactones kawain (K), desmethoxyyangonin (DMY), methysticin (M), dihydromethysticin (DHM) (each at 10 microM), and NADPH. Whereas K did not inhibit these enzymes, there was significant inhibition of CYP2C9 by DMY (42%), M (58%), and DHM (69%); of 2C19 by DHM (76%); of 2D6 by M (44%); and of 3A4 by DMY (40%), M (27%), and DHM (54%). Consistent with their potency as inhibitors, the two major kavalactones bearing a methylenedioxyphenyl moiety (M and DHM) formed ""455 nm"" metabolic intermediate complexes after incubation with HLMs and NADPH, but K and DMY did not..""