I wouldn't dare take that much B12 with everything else going on in my body.
When taken at appropriate doses, vitamin B-12 supplements are generally considered safe. While the recommended daily amount of vitamin B-12 for adults is 2.4 micrograms, higher doses have been found to be safe. Your body absorbs only as much as it needs, and any excess passes through your urine.

High doses of vitamin B-12, such as those used to treat a deficiency, might cause:

Headache
Nausea and vomiting
Diarrhea
Fatigue or weakness
Tingling sensation in hands and feet