Not seen any credible reports about it being addictive, but I suppose that it would be possible to become psychologically dependent on it. That can happen with pretty much anything. If you are worried about it being toxic go for a small dose and don't take it for months on end.
@Sarah Metcalf I've heard people say they like taking kava kava. I've tried it a few times, even taking only half a capsule before retiring. It made me feel drugged, sort of "knocked out", and I had a strange drugged feeling for all the next day, and into the day after. I don't remember the name of the brand or the amounts in the capsules.
I can't say anything about its effects on the liver. Sorry.