On cipro (ciprofloxacin) for kidney infection had reaction so was changed to bactrim. Now having rash, shaking so switching it agin. Is it safe to keep switching?

Adverse reactions. Unfortunately, it IS necessary to change if now you're getting a rash reaction to Bactrim (sulfamethoxazole and trimethoprim) - which is a 'sulfa' drug. Knowing what the germ is and it's sensitivity profile would help select the right antibiotic for you -- as well as keeping in balance with your allergies/medical history. Usually I like to treat for 10 or 14 days for a kidney infection- so hopefully not that much treatment left..