I used bactrim for 15 days a few months ago. It cleared most of my acne by the end of this period; however, my acne came back right after. Also, I developed flu-like symptoms 5 days in. When I took bactrim a second time a month later, my ears started ringing and I felt sick. Turns out I'm sensitive to sulfamethoxazole (which is part of bactrim). If you have ANY side effects or 'coincidental' sicknesses during the bactrim, I would suggest stopping it immediately as you can cause permanent damage (to your hearing).
I used Bactrim back in 2011 - I think I used it for two periods of 2 months each that year, in tandem with Ziana gel. It was pretty helpful and cleared a lot of my spots away, iirc, but it was definitely not a long-term fix. Pretty much right after stopping using it, my acne came back in a torrent. Plus, I would still get flare-ups while using Bactrim/Ziana (lasted maybe 1-2 days at most every so often).
I've had much, much more success with Ceftin / Cefuroxime Axetil and Clindamycin.