What Are Side Effects Of Taking Hifenac MR?

Are there any serious side-effects of taking Hifenac MR? My mom who is 75 years old, was relieved of any uneasy, tingling, sensation in her feet but has an extremely bad taste in the mouth since she started taking it. She is wondering if the drug is safe and should she continue it. she has extreme arthritis, is mildly diabetic (on small dose of drugs) and manages it with her diet. She has no physical activity, has extreme pain in the knees and her knee replacment is not possible so she suffers in bed, goes to places where it is necessary to go