According to Harvard Medical School, a vitamin B12 deficiency can occur if a person is either not consuming enough vitamin B12 or her body cannot properly absorb the nutrient. A vitamin B12 deficiency can range from mild to severe and can have symptoms such as fatigue, jaundice and weight loss.
Meat, seafood and animal products contain vitamin B12, so most people receive enough vitamin B12 through their diet. Vegans and vegetarians are at risk for vitamin B12 deficiency and may need to consume additional supplements. WebMD states that some people have pre-existing conditions that prevent the absorption of certain nutrients; these people may need to utilize supplements, prescriptions or injections to avoid vitamin B12 deficiency.