Well... there are posts on the net discussing antidepressants losing effectiveness. Seems to be quite common.

But I think that Cymbalta is not a great med to begin with. It has been hyped and promoted, but not many people "like" it or have success with it.

Drugs that work on neurotransmitters, which are FDA approved, are not really well understood as to how they work. Because of this there is still no real understanding of how Cymbalta works on pain (if it does at all).

There are some theories that drugs that work on receptors, over time induce the cells to make MORE receptors. This is thought to be a survival mechanism. So that when the drug is stopped it leaves MORE receptors behind, so that if you restart the drug later in time, you have to fill more sites to get effects, hence needing larger doses.

This has been demostrated with opioids, that the glial cells in the brain stimulate further mu receptors over time. I think this may hold true for other drugs that affect neurotransmitter receptors and levels.

I think most people do not understand how little is known about powerful drugs like Cymbalta and other antidepressants. This is also true of gabapentin and Lyrica. It took many years, and many patients to have difficulties with withdrawal because of this lack of knowledge.