because the drug stops working (very common for antidepressants.)

or because they develop liver toxicity (which this drug causes, and a black box warning is pending for this in fact)

or because their blood pressure has become high and is due to this drug.
(both Effexor and Cymbalta raise blood pressure)

or because a newer drug has come out that is better, more effective and less toxic. Prepare for the introduction of Pristique (Wyeth) soon.

OR because it is NOT working as planned. (most common)

Or the patient can no longer afford it.

(according to some studies-- mostly by Lilly~~ up to 47% of patients experience unpleasant withdrawal events when
coming off Cymbalta)

SSRIs and SSNIs cause withdrawal because they alter neurotransmitters in the brain. That is, THEY CHANGE you. Just like narcotics.
The only difference is that narcotics work on different receptors. SSRIs and SSNIs were put on the market after short
studies, and long term use was never investigated. It took years to realize what was really happening.
And today we still have clouded information as to how they really work. (most of theories are just that..not fact).