Thanks for contacting HCM From your question I understand you are concerned about possible drug interactions between Bactrim and Tri Sprintec. When I have patients on birth control I recommend that if they are taking an antibiotic with their birth control that they add a additional birth control method like condoms. Antibiotics can interfere with the birth control and make then less effective. It sounds like you were concerned enough to be worried about it too. Withdrawing from you during orgasm is not the most efficient way to prevent pregnancy. If you do engage in sex while on Bactrim I would recommend using a condom. Hope this answers your question. If you have any further questions please contact us again