Gum swelling and pain may be caused by periodontitis or periapical periodontitis, taking metronidazole has a certain therapeutic effect, but the specific time of effect varies from person to person, usually about 5-7 days, need to be combined with the corresponding periodontal treatment and root canal treatment, in order to completely eliminate inflammation. 1. Periodontitis: Metronidazole has a certain therapeutic effect on periodontitis, which can kill gram-positive and negative anaerobic bacteria and relieve inflammation of periodontal tissues. Simple medication can not completely cure swollen and painful gums, and need to be combined with periodontal treatment, such as supragingival cleaning, subgingival scraping, root planing and so on. 2. Periapical inflammation: Metronidazole can reduce periapical inflammation by killing anaerobic bacteria. Combined with root canal treatment, it can completely eliminate inflammation in the root canal and periapical area. If the swelling and pain of the gums are obvious and the surface appears to be fluctuating, the gums can be incised at the same time to drain the pus. The use of drugs should be carried out under the guidance of a doctor, there may be other causes of swollen and painful gums, it is recommended to go to the hospital in time to receive regular treatment.