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Bactrim is primarily used for urinary tract infections which are caused by bacteria significantly different than the type typically infect teeth. As a rule bactrim is ineffective against tooth abscesses. You need to take an antibiotic in the Penecillin (Amoxicillin) family of Erythromycin (Clindamycin) family of antibiotics in order to help with your abscess.
Call your dentist's office and he should be able to call in a prescription for you.
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