RE: Kava
I believe that a more accurate comment might be that long-term use of excessive amounts of highly potent Kava "may" cause liver problems.
Most of the Kava sold in the US (capsules thru health food stores) is junk and very weak. You could take their recommended doses every day forever and not experience any problems. The potent stuff (usually need to buy from Hawaii or other sources where you're obtaining quality potency) works well but should be used only periodically.
But then if you were using it for anxiety and still had the anxiety three months later, it'd probably be time to go talk to a psych doc anyway, no?
But for periodic use, or just to "mellow out" occasionally, no problem.
Some books on the subject:
Kava : nature's stress relief / Kathryn M. Connor and Donald S. Vaughan.
Kava : the miracle antianxiety herb / Ray Sahelian
Kava : the ultimate guide to Nature's anti-stress herb / Maggie-Greenwood Robinson.
Kava : nature's answer to stress, anxiety, and insomnia / Hyla Cass, and Terrence NcNally.
Kava : nature's relaxant for anxiety, stress and pain / Hasnain Walji.