If bad breath is caused by an anaerobic infection, taking metronidazole will be effective. Conversely, if the treatment of bad breath with metronidazole is effective, it may be caused by anaerobic bacterial infection. If the bad breath is caused by dental plaque or other oral diseases, treatment with metronidazole is not effective, you need to go to the dental hospital to clean teeth, or other treatment. Bad breath may also be caused by H. pylori infection, treatment with metronidazole is also ineffective, need to use H. pylori infection eradication treatment to treat bad breath.