There are many reasons for the pain in the upper left big tooth, considering that it may be caused by pericoronitis, deep caries, pulpitis, periodontitis, apical periodontitis and other reasons. 1. Peri-coronitis: If the upper left big tooth is a wisdom tooth, it is possible that food residues are left in the gingival sulcus due to inadequate cleaning, leading to inflammation of the gums around the crown, which causes pain in the big tooth, and anti-inflammatory drugs can be taken to relieve the pain. 2. Deep caries: When the decay of the upper left big tooth reaches deep caries, the tooth may be sore and sensitive when it encounters hot and cold stimuli as well as overly sweet and hard food, which needs to be filled in time. 3. Pulpitis: After the decay of the upper left tooth reaches the pulp cavity, the pulp is infected by bacteria and becomes congested, so the tooth will experience spontaneous, paroxysmal, radiating severe pain, which needs to go to the hospital for root canal treatment to relieve. 4. Periodontitis: If the upper left tooth has too much calculus that presses on the gums and causes periodontitis, the tooth will be swollen and painful, bleeding, loose and other symptoms, and the tooth needs periodontal therapy to treat it. 5. Periapical periodontitis: If the decay of a large tooth is not treated in time, the pathogenic bacteria and infectious substances ferment in the pulp cavity and progress along the root canal to the root tip, thus forming a periapical periodontitis resulting in tooth swelling, pain, biting pain and other discomforts, which requires root canal treatment to relieve the pain. There may be many other possible causes of pain in the upper left large tooth, and you should seek medical attention to identify the cause and standardize treatment under the guidance of your doctor.