First of all. Sunny is totally incorrect on all counts. Valium and Valerian have absolutely NOTHING in common other than a similar sounding name. They share no chemistry, no similar structure, no components in valerian are even similar looking to valium. Benzodiazepines are totally synthetic and occur nowhere in nature. And hepatotoxicity does refer to liver damage, not kidneys. Kidneys are the renal system, liver is the hepatic system.
Okay. This is not clear at all. Part of it is that the current laws in the US for the manufacture and sale of anything labeled a supplement are pretty abysmal, inferior to the standards prison food is kept to really. So it is a distinct possibility that the cases of liver damage related to valerian are due to contaminants - companies are basically accepted on their word at quality control and some very nasty things have been found in these products, everything from rat poison to feces to human placenta.
On the other hand, some in-vivo data published in Xenobiotica last year done by the University of Riyadh shows valerian is capable of massively inhibiting an enzyme system in the liver called UGT. On paper, this is incredibly bad. UGT is responsible for something called glucuronic acid conjugation - and this is basically how your liver gets rid of a huge amount of metabolic waste, basically all the molecules your body wants to get rid of go through this system from aspirin to your thyroid hormones. You can see how this could conceivably be very bad. People born with genetic defects in this pathway have everything from chronic jaundice, metabolic abnormalities to recurrent developments of different kinds of cancer.
However, despite how bad that looks on paper, the real world is much brighter. The UGT system has hundreds of times the capacity it needs to handle your metabolic waste. So in the real world, we don't really see any of this kind of horrible problem. But it -does- make it plausible for Valerian to be toxic to the liver. It's not -definitely- toxic like say, Kava Kava, but it could be - there's just no way to tell because there's no large scale tracking of this sort of thing - the cases on record could be due to valerian being toxic, or could be due to the low standards of manufacture and frequent contamination. There could be a lot of cases of damage no one knows about, because again this isn't kept track of.
Based on the widespread use, even with the poor study and tracking, Valerian in normal use doesn't liver problems very frequently at all (that we can tell). In high doses - well it works on paper, but it's not certain in the real world. Probably shouldn't take the chance, because it can't be said for certain that it doesn't.
Feel free to email me privately if you need anything clarified