When you study herbs, they have a general warning about teh ones like Milk Thistle and Comfrey that actually work on cells rather than systems. See, most herbs energize and cleanse by working on a system, like Echinacea promotes the entire lymphatic system to cleanse. You don't want to overdose there or you continually ramp up the system, which over time is tiring and draining, just like any kind of exercise.
With herbs that work on a cellular level, they ramp up healing and reproduction of the cells themselves, not the entire system. The more cells reproduce, the higher the chances of creating a bad cell, either as a benign cyst or cancer.
The chances of this are really rare, so don't let it worry you, especially since most MT supplements don't have a lot of the active ingredient anyways! Most people don't take enough to feel the herb working (detox headaches most common side effect) so you're really not doing all that much. I'm just very cautious when it comes to prescribing things, as we've all had bad experience with doctors throwing drugs at us without respecting them or knowing exactly what they do. Herbs can be just as strong as drugs.