Side effects may occur in 80-year-olds taking methylcobalamin, but the side effects of methylcobalamin are generally not related to the age of the patient using the drug. Anyone of any age taking methylcobalamin may experience gastrointestinal side effects, such as loss of appetite, nausea, and skin rashes. 1. Gastrointestinal adverse reactions: some patients may experience nausea, vomiting, diarrhea, and loss of appetite. 2. Allergic reactions: a few patients may experience allergic reactions, such as rash and itching. When taking methylcobalamin, it should be noted that if there is no obvious effect for more than one month, it is recommended to stop using it; it is not suitable for long time to take it for a long time for staffs who are engaged in mercury and its compounds for a long period of time; if it is opened later, it should be avoided from light and humidity. It is recommended that elderly patients need to use the drug under the guidance of the doctor, unreasonable use of the drug may cause aggravation of the condition, but also may aggravate the adverse effects of the drug.