Hi, Thanks for your question. After going through your question, I can understand your concern.. Tab Marifur contains Nitrofurantoin. Usually Nitrofurantoin is very well tolerated medicine. May have side effects like nausea, vomiting, abdominal pain, diarrhea (very rare to have constipation), peripheral neuropathy, allergic reaction. So your constipation may not be related to use of Marifur. Reading your question there is possibility of resistant urinary tract infections in your case, in such case I would like to advise - - Evaluation for any structural abnormalities in urinary system. For that Ultrasonography of abdomen and pelvis is advisable. - Urine examination including culture and sensitivity. Depending upon these tests type of appropriate antibiotic and duration of treatment can be decided. Hope this helps you, if so do vote as helpful.