Usual disclaimer.. I'm not a doctor or medical professional, please do your own research and consult with a doctor.
If you do some google searches, it seems that kava kava was only toxic on the liver when leaves of the plant were used. As far as I am aware there are no damaging effects listed from using the root alone, which contains the greatest concentration of kavalactones in the plant. Make sure you order from a reputable supplier. I've used konakava.com in the past and was very happy