Methylcobalamin is commonly used in neurology as a nutritive nerve medication, and when taken improperly may have adverse effects such as nausea, vomiting, loss of appetite, diarrhea, allergies, and skin rashes. Taking too much can also lead to liver and kidney impairment. Methylcobalamin is a derivative drug of vitamin B12, which can participate in the biochemical synthesis process of brain cells and spinal cord nerve cells, and has the effect of promoting nerve axonal transport. Approximately 0.1%-5% of patients experience adverse gastrointestinal symptoms such as nausea, vomiting, loss of appetite, diarrhea, etc. A very small percentage (<0.1%) of patients also develop a rash. The drug should be taken according to the dose, not too much at once, otherwise it will cause damage to the liver and kidney function. It is also important to note that the drug is contraindicated in patients with a history of allergy to methylcobalamin and its excipients. To summarize, methylcobalamin should be taken under the guidance of neurologists, and should not be taken blindly. If any discomfort occurs in the course of taking the drug, you should go to the hospital as soon as possible.