long term herbal supplementation

I just have a quick two part question about taking herbs like goldenseal or echinacea.

In all the research a read and warnings it always says don't take these supplements more than a few weeks but I can never come up with a reason as to why. How could they be harmful after long term use and what areas of the body could they be having a negative effect on?

Second part is when after stopping can I begin again? I been taking goldenseal and seeing great results but I stopped after 4 weeks. Tweet