The side effects of methylcobalamin are as follows: 1. Patients may experience gastrointestinal reactions, such as bloating, diarrhea, indigestion, loss of appetite, etc. 2. Dizziness may occur. 3. Some patients may experience allergies, such as rash and itchy skin. The overall side effects of methylcobalamin are relatively mild. Once serious side effects occur, it is recommended to stop the drug and go to the hospital for timely treatment. Methylcobalamin is actually an oral dosage form of vitamin B12. Normally, vitamin B12 should be injected intramuscularly. Vitamin B12 mainly nourishes the nervous system and has good therapeutic effects on symptoms such as nerve demyelination changes, neuritis and hand numbness.