For +15 yrs I've taken 6x 500mg paracetamol (tylenol) for my headaches at a time. Up to 21 tabs a day if really bad. How bad is it for my liver?

Very!!! Maximum dose used to be 4, 000mg/d but has recently been lowered to just 3, 000mg/d. Well, at least here in the states. I have no idea what is allowed in the uk but 10, 500mg/d can't be good. Check out http://www.Consumerreports.Org/cro/news/2011/11/repeatedly-taking-too-much-tylenol-can-be-dangerous/index.Htm. Then go see your doc immediately for testing. Good luck! don't keep rolling dice!