Methylcobalamin tablets are an oral prescription drug, and the adverse reaction item in its instruction manual suggests that 146 (0.96%) adverse reactions occurred out of 15,180 total cases. (At the end of the investigation of the frequency of adverse reactions) Gastrointestinal: occasional (5%-0.1%) loss of appetite, nausea, vomiting, diarrhea; allergy: rare (<0.1%) skin rash. A study in the literature showed that in the efficacy of using methylcobalamin tablets in 30 patients with postherpetic neuralgia, the total efficiency of patient treatment was 80.0%, with one case of nausea and one case of diarrhea, and the incidence of adverse reactions was 6.7%. A study in the literature showed that in the observation of the effect of using methylcobalamin tablets on 43 cases of treatment of diabetic peripheral neuropathy, there were 2 cases of mild nausea, 1 case of vomiting, 2 cases of dizziness, 1 case of middle and upper abdominal discomfort, and 1 case of swelling, and the incidence of adverse reactions was 16.28% (7/43). All adverse reactions disappeared automatically without treatment. In conclusion, the side effects of methylcobalamin tablets include loss of appetite, nausea, vomiting, diarrhea; rash. Patients should take the drug as prescribed by their doctors. Inform the doctor promptly if new side effects are found during the use of the drug.