Most cases of swelling on one side of the anus are due to perianal inflammation and perianal abscesses. Since the anus is the exit for excreting stool, the anal sinus and the area around the anal mucosa are weak and therefore very susceptible to bacterial contamination and the formation of red and swollen lumps and related symptoms. If you have painful swelling around the anus, you must go to a regular hospital for a detailed examination because once you have a perianal abscess, you need to undergo surgical incision and drainage, and you must not handle it at home by yourself so that the abscess formed around the anus does not continue to break down and lead to the formation of an anal fistula, which will have a great impact on the future surgery and the recovery of the perianal abscess. It is best to visit the hospital in time to find the exact cause of the swelling on one side of the anus for thorough treatment.