I was diagnosed with bv. my doctor prescribed me bactrim (sulfamethoxazole and trimethoprim). will it work? when will it take affect?

Atypical treatment: Trimethoprim-sulfamethoxazole (Bactrim®) is not a recommneded treatment for bacterial vaginosis and probably will not work. The usual treatment is metronidazole (Flagyl and other brands) or tinidazole (Tindamax), sometimes clindamycin. Discuss with your doctor; or if s/he is not highly familiar with BV and its treatment, maybe get a second opinion. Good luck.