Vegans (vegetarians who also avoid dairy and eggs) frequently become deficient, though the process often takes many years. People with malabsorption conditions, including those with tapeworm infestation and those with bacterial overgrowth in the intestines, often suffer from vitamin B12 deficiency. Malabsorption of vitamin B12 can also result from pancreatic disease, the effects of gastrointestinal surgery, or various prescription drugs.¹³

Pernicious anemia is a special form of vitamin B12 malabsorption due to impaired ability of certain cells in the stomach to make intrinsic factor—a substance needed for normal absorption of vitamin B12. By definition, all people with pernicious anemia are vitamin B12-deficient. They require either vitamin B12 injections or oral supplementation with very high levels (1000 mcg per day) of vitamin B12.

Older people with urinary incontinence¹⁴ and hearing loss¹⁵ have been reported to be at increased risk of B12 deficiency.

Infection with Helicobacter pylori, a common cause of gastritis and ulcers, has been shown to cause or contribute to adult vitamin B12 deficiency. H. pylori has this effect by damaging cells in the stomach that make hydrochloric acid—which is needed for normal absorption of vitamin B12. In one trial, H. pylori was detected in 56% of people with anemia due to vitamin B12 deficiency. Successful eradication of H. pylori led to improved blood levels of B12 in 40% of those infected.¹⁶ Other studies have also suggested a link between H. pylori infection and vitamin B12 deficiency.^{17, 18} Elimination of H. pylori infection does not always improve vitamin B12 status. People with H. pylori infections should have vitamin B12 status monitored.

In a preliminary report, 47% of people with tinnitus and related disorders were found to have vitamin B12 deficiencies that may be helped by supplementation.¹⁹

HIV-infected patients often have low blood levels of vitamin B12.²⁰

A disproportionately high amount of people with psychiatric disorders are deficient in B12.²¹ Significant vitamin B12 deficiency is associated with a doubled risk of severe depression, according to a study of physically disabled older women.²²

A preliminary study found that postmenopausal women who were in the lowest one-fifth of vitamin B12 consumption had an increased risk of developing breast cancer.²³

Although blood levels of vitamin B12 may be higher in alcoholics, actual body stores of vitamin B12 in the tissues (such as the liver) of alcoholics are frequently deficient.^{24, 25}

Low blood levels of vitamin B12 are sometimes seen in pregnant women; however, this does not always indicate a vitamin B12 deficiency.²⁶ The help of a healthcare professional is needed to determine when a true vitamin B12 deficiency exists in pregnant women with low blood levels of the vitamin.

Hydroxocobalamin (a form of vitamin B12) has been recognized for more than 40 years as an effective antidote to cyanide poisoning. It is currently being used in France for that purpose. Because of its safety, hydroxocobalamin is considered by some researchers to be an ideal treatment for cyanide poisoning.²⁷