Kava kava is an excellent natural muscle relaxer and sleep aid. You would have to abuse it severely to have any kind of negative side effects from it. Sounds like it was cutting into some pharmaceutical company's business.

Here is an article I wrote about natural sleep aids. Kava Kava is given a brief mention:

Sleep Without Drugs
http://hubpages.com/hub/Sleep-Without-Drugs

Kava? Yeah, tried it when in Fiji. You can find out the health problems elsewhere but as far as I'm concerned it's a great drink. Tastes like muddy water, makes your tongue and lips go numb, and makes you fall asleep easier. It also is a great social drink, loosening your tongue a lot slower than alcohol might. I think its one of those things that had it been present in England for the last few hundred years, it'd be perfectly legal.

Piper methysticum

Indications: anti-anxiety, sedative, antispasmodic, anti-convulsant.

Contraindications: Endogenous depression as it increases the danger of suicide. Also contraindicated in pregnancy and nursing mothers.

Side effects: An allergic reaction may manifest as yellowing of the skin. Rare accomodation disturbances such as enlarged pupils, GI complaints and occulomotor disturbances. There's way more info out there but my source was: www.quickstudy.com

It's not bad -- you'd have to drink it a LOT and for a long time to get any sort of liver damage.