Gums with foul odor when pressed may be caused by periodontitis, as follows. Periodontitis is a chronic infectious disease caused by plaque biofilm, and its clinical manifestations include intraoral odor. The periodontopathogenic bacteria in deep periodontal pockets have a strong odor-producing ability, and the main source of their odor is methyl mercaptan. Together with endotoxin and interleukin, methyl mercaptan can stimulate the production of prostaglandins and collagenase, further aggravating periodontal lesions. In addition, patients with periodontitis have a more coated tongue, and the food debris and shed cells that remain on the back of the tongue become a substrate for bacterial degradation, which is also a major source of oral odor. There may be other reasons why the gums are smelly when pressed, so it is recommended to go to the hospital in time to clarify the cause of the disease and then carry out targeted treatment.