Bactrim (co-trimoxazole) is a prescription medicine intended for patients that developed some bacterial infections but are unable to deal with it without taking some medications of that kind. Bactrim needs to be taken regularly and the entire amount of this medication should be used. If you stop the treatment before it's time, a relapse of infection is possible and you may need to take other medications to treat the infection. Make sure you let your health care provider know if you are using any diuretics, methotrexate, ACE inhibitors, blood thinners, or seizure medications, as those may be likely to cause interactions and their doses will need to be adjusted. Mild side effects include loss of appetite, dizziness, upset stomach, joint pain, swollen tongue, spinning sensation, sleep problems, and vomiting. There is no need to report them to your health care provider, as they are most likely to go away on their own. More serious side effects will need to be reported to your health care provider right away and include sore throat, fever or chills, joint pain, yellowing of the skin or eyes, paleness, itching, mouth sores, unusual bruising or bleeding, or skin rash.