Mom was diagnosed with clinical depression back in the 1970s, but for years was fine when she worked several jobs. However, she went back into a severe depression in 2008 when my job was transferred 2 hours away and my aunt died within two weeks of each other. I had to hospitalize her but she never totally came back completely, and at that point I noticed that her speech had considerably slowed down and began to slur. Had her checked for strokes but she did not have them. She does have Paget's disease of the bone which is in her skull, and she is almost completely deaf in the left ear.
Rule out physical causes before assuming mental conditions. None of us want to lose our rights prematurely.
http://en.wikipedia.org/wiki/Charles_Bonnet_syndrome is one reference to check out.
Might she have a UTI?
How is her vision? Has she been checked for maculsr degeneration, etc?
Have another Dr. check, perhaps a gerontologist. A second opinion is always a good choice.
That being said, it's good to learn about successes with this med and the others that aren't, etc.