Is It Safe To Take Celexa Instead Of Zoloft?

My mother-in-law who is 86 just started on Zoloft two weeks ago, but we have not seen any improvement yet. She had been having some nervousness, days were she would be upset and cry, and some confusion. Was wondering if she should switch to celexa or a different medication or should we give it more time. She complained yesterday that she was shaky all day. She has not been eating well and we have been encouraging her to eat. She did eat today and seemed better.