A lack of B12 may lead to depression, confusion, memory problems, and dementia. It also can affect your balance. B12 supplements are usually safe. For adults, doctors recommend 2.4 micrograms a day. If you take more than what you need, your body passes the rest out through your pee. Still, high doses could have some side effects, like dizziness, headache, anxiety, nausea, and vomiting.