Long term use of MiraLax

Does anyone know  why MiraLax should not be used long term? I have been using it for probably 6 years on my kitty.

I didn't know that there were any reasons to not use it long term.  If it worked like a more traditional laxative then I would understand not using it long term.  

Thanks for your thoughts, Denice.  I keep seeing statements that say that MiraLax should not be used long term and my Maine Coon has been on it for at least 5 years, so I wondered if anyone else had read the same thing...no one ever says why it should not be used long term.