Hello, Yes, Septrin (also known as Bactrim or sulfamethoxazole/trimethoprim) can have side effects, including allergic reactions such as skin rash, itching, hives, and swelling of the face, lips, tongue, or throat. Other common side effects include nausea, vomiting, diarrhea, and loss of appetite. Rare side effects include fever, chills, sore throat, mouth sores, easy bruising or bleeding, and unusual weakness. As for your weight problems and fluid retention, these could be related to a number of factors, including diet, exercise, medications, and underlying health conditions. It is possible that your reaction to Septrin could have contributed to these issues, but it would be best to consult with your doctor for a thorough evaluation and treatment plan. Insulin resistance is a condition in which your body s cells become less responsive to insulin, which can lead to high blood sugar levels and eventually diabetes. While Septrin is not known to cause insulin resistance directly, certain medications, lifestyle factors, and underlying health conditions can increase your risk. Again, it would be best to discuss this with your doctor to determine the underlying cause and appropriate treatment plan. You might require to run further tests for this purpose. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Albana Sejdini, General & Family Physician