Although niacinamide can indeed be toxic to the liver, this doesn't necessarily mean it is unsafe to take. A Swedish medical school review of the safety of niacinamide found:
"... at very high doses reversible hepatotoxicity has been reported in animals and humans. Minor abnormalities of liver enzymes can infrequently occur at the doses used for diabetes prevention." http://www.ncbi.nlm.nih.gov/pubmed/11126400
The key points here are that only VERY HIGH doses can cause hepatoxicity, and any damage is REVERSIBLE, so your liver recovers when you stop taking it. What is considered a VERY HIGH dose? Well the above review said the doses used to prevent diabetes (typically 1500-3000mg) only INFREQUENTLY cause MINOR ABNORMALITIES. So I think we can safely assume the dose needed to produce significant hepatoxicty would need to be at least 3000mg/day.
So, as far as liver toxicity is concerned, I see little or no risk at doses up to 500mg, slight risk 1000-3000mg, and significant risk at more than 3000mg. Of course the risk increases if you are taking other medications or supplements that stress the liver. If in doubt I recommend having regular liver enzyme tests. Although niacinamide may not cause any problems for your liver, it is best to be safe than sorry.