What in particular about methysticin is concerning to you in that report? It seems to show that methysticin is beneficial in certain aspects, and I don't see anything that suggests toxicity.
> methysticin was identified as a potent NF-kappaB inhibitor with minimal
toxicity
> The authors suggested that methysticin might be responsible for kava’s chemopreventative efficacy.
This suggests that methysticin is anti-inflammatory and anti-cancer, though this is a reductive statement for sure.
> Kava
extract, methysticin and DHK protected
brain tissue against ischaemic damage in
experimental models.
Methysticin has a protective effect your brain in situations of inadequate blood flow.
It's always good to be cautious about the health implications of anything you eat or put into your body. I am as well. But currently available evidence suggests that kava is quite safe, and even beneficial. I would like to see more controlled studies with aqueous kava preparations (i.e. the traditional drink) for more clear data though.