Adults using vitamin B-12 to improve their mental performance commonly receive a 1,000 microgram injection every day for five days, followed by a monthly 1,000 microgram injection for five months, reports Mayo Clinic. The recommended dietary amount of vitamin B-12 is 2.4 micrograms daily.
Generally, vitamin B-12 is considered safe when the recommended daily amount is taken. However, vitamin B-12 supplements sometimes cause complications in people with cancer, skin disorders, gout, high blood pressure or heart conditions. Vitamin B-12 supplements also interact with certain medications, including aspirin, antibiotics and oral contraceptives, reports Mayo Clinic. In addition, some people are allergic to vitamin B-12.