Though there have been some credible studies, it's unfortunate that Kava, like so many other therapeutically/medicinally relevant plants due to government and pharmaceutical disinterest or counter-campaign, is lacking in the empirical research/evidence department.
IME, either the primary kavalectones or more likely their secondary/tertiary metabolites have an extremely long duration of action, which remind me of valium. With both diazepam and Kava, I will only take a moderate dose on, say a Friday evening, because of the "hangover" it leaves me with on Saturday- just an all around dull groggy feeling where my eyes are visibly hazy and not all the way open, I look drowsy and out of it, and I feel irritable. These effects persist throughout the following day. For this reason I rarely take it, despite very much enjoying its effects and therapeutic benefits. As mentioned, it is quite unique and pleasant, and especially noteworthy due to its unrestricted status as an herbal supplement.
It may be worth noting that I have a high quality, high potency [standardized] capsule formulation of Kava. When I used to partake in kava sessions, we'd drink prodigious amounts of brew made from potent strains my friend brought back from Fiji, and I don't recall ever having any hangover-type effects the next day. This suggests the method of consumption may play a significant role in its subjective effects.
