###Human: Why am I having swollen gum behind last tooth with cold? ###Assistant: Hello. I have been through your query and I can understand your concern. The swollen gum behind your tooth is a condition called pericoronitis. It generally occurs with the partially erupted last molar or teeth. The food gets entraped between the tooth and gum and it gets infected leading to pain and swelling of the gums. It is also characterized by the pain while opening the mouth. After checking your drug history I would advise you to take antibiotics Metronidazole 400 mg three times daily for three days and also analgesic Aceclofenac 100 mg (Zerodol MR) when you have any pain (if you are not allergic to any of these). Also use Hydrogen Peroxide 0.1 % mouthwash diluted in water (1:1) three times daily for five days. If the problem still persists, I suggest you go to the dentist and get the procedure called operculectomy done. Also, I would advise you to have good oral hygiene from now on and brush your teeth twice daily and floss regularly. Use mouth rinses after every meal.