Methylcobalamin tablets are commonly used to treat peripheral nerve diseases, with fewer side effects. The following are possible side effects of long-term use: 1. Gastrointestinal reactions, a relatively common side effect, are likely to occur when the drug is first used, often manifesting as loss of appetite, reduced eating, sometimes nausea, and in severe cases, patients may have vomiting or diarrhea, and patients who cannot tolerate it can stop using the drug. 2. It is only seen in people who are allergic to methylcobalamin, which can cause rash, and patients with allergic history should be prohibited from using it.3. For people who are exposed to mercury and its compounds, they should avoid using the drug for a long time, and if the patient has no obvious effect for one month, it is also recommended to stop using the drug. This drug is an endogenous coenzyme B12, which can promote the synthesis of nerve myelin and the regeneration of axons, and can be used for a long time, but the specific course of treatment should be carried out according to the doctor’s recommendations.