JD, You said AD's don't stop working. Clearly, they do stop working in some cases, apparently with enough regularity that they do study this as an issue for some patients. Another study said this isn't a hugely common problem, but it does seem to occur in about 8% of the cases. The doctors use "poop-out" simply because that is the colloquial/layman's term that has been used by patients in the past, and those particular articles were for patients, not doctors. Do they know why it happens? No, not clearly. It may be a number of factors, including some genetic factors. It is a real problem for some patients, myself included.