Is it harmful to take methotrexate for Rhematoid Arthritis?

My dentist said that Methotrexate was very bad for me: that it "kills everything." My recent blood test showed a decrease in kidney function. Is it due to the methotrexate. I am not sure if I even have RA because all of the tests for it have been inconclusive and I do not have many of the signs in that all of my symptoms seem to be only on my right side. I do have occasions when I am unable to make a fist. I should add that I have hypertension and diabetes. I take medication for both these conditions. Thank you for any help you can give me.