When administered systemically via oral tablets, Metronidazole works by inhibiting bacterial DNA synthesis hence preventing the replication of bacteria. Stopping the medication too early may allow bacteria to continue to grow resulting in a relapse of the infection. A day for ten days or longer. Metronidazole should not be used in the treatment of many protozoal and bacterial infections. Flagyl or Metronidazole is the effective modern antibiotic.