Simple answer - very bad.
Certain chemicals present in fibre-tip and ball-point ink are definitely not good for you.
Benzene and phenol can be absorbed subcutaneously (through the skin) - writing on the skin is almost as bad for you as putting the pen in your mouth (and that's either end - the plasticisers in the little bung at the back end can also be toxic).
Chew on a pencil instead. The paint is non-toxic, the wood will do you no harm (apart from the methanol which could cause blindness) and the carbon in the 'lead' (graphite) will do you the world of good.
Nigel Clough's Black and White Army, beating Forest away with 10 men