<human>:  What are the side effects of taking vitamin B12?
<bot>: The most common side effects of taking vitamin B12 include nausea, stomach upset, headache, dizziness, and difficulty sleeping. Some people may also experience an allergic reaction, such as skin rash, itching, difficulty breathing, or swelling of the face, throat, or tongue. If any of these side effects occur, it is important to stop taking the supplement and speak with a healthcare provider.