Berstein (2007) does have a comment re: niacin. I have no idea at what levels or anything else - just reporting it for your info.

"...reports of liver toxicity [from rosiglitazone and pioglitazone] are far fewer than with some commonly used medications such as niacin and the so called statins..." (p.244)

He goes on to suggest for these meds at least [the ros- and pio-] that it is a good idea to have your blood tested annually for liver enzymes.

This may be old news to you but thought I should post it anyway.

/smile