If you are taking a prescription medicine that is known to stress the liver, then you should be very careful about taking supplements such as green tea extract and niacin. Both green tea extract and niacin can be hard on the liver. In fact my doctor says that most herbs and herbal extracts can elevate liver enzyme levels. (And of course you have heard the ongoing news about the dangers of taking too much Tylenol.)
If you are taking a medication that is known to stress the liver, and you want to take green tea extract or niacin (or other supplements), then discuss your plans with your doctor and have your liver enzyme levels tested.