Before taking a vitamin B-12 supplement, it is best to consult a physician for assistance in determining the correct dosage, advises the University of Maryland Medical Center. The recommended daily adult dosage is 2.4 micrograms. In most instances, vitamin B-12 is safe and nontoxic.
Taking any one of the B vitamins for an extended period of time can result in an imbalance of other essential B vitamins. For this reason, it may be useful to take a B complex, which includes all B vitamins, recommends the University of Maryland Medical Center.
Some medications reduce B-12 levels in the body. These include antiseizure medicines, such as phenytoin, phenobarbital and primidone; chemotherapy agent methotrexate; gout remedy colchicine; and H2 blockers, such as cimetidine, famotidine and ranitidine. An individual taking any of these should consult a physician before taking a B-12 supplement to avoid possible interactions.