bitters don't have any effect when administered rectally, i am 99% sure. these can be taken orally but must be tasted for them to work.
in the case of coffee enema it is partially the caffeic acid that simulates the release of bile and an increase in glutathione.
for some people more is more. if you don't have sufficient amount of coffee then you will just stimulate and not release bile. too much could possibly be overstimulating, BUT..
the thing to remember, and why not to be afraid of coffee enemas is that the whole point is to release stagnant bile, which will overall make you feel better, even in the case of adrenal fatigue. if you do a Coffee-Enema and do not release bile you will either feel nothing or you will feel worse.
releasing stagnant bile or stones is a major part of getting well from any illness and why the Liver Flushes are so popular.