Vitamin B-12 is taken when a person has a vitamin B-12 deficiency, according to WebMD. Vitamin B-12 is also taken to treat conditions such as Alzheimer's, high cholesterol, breast cancer and sickle cell disease. A doctor often recommends vitamin B-12 to treat heart disease.
People over the age of 50 are most likely to have a vitamin B-12 deficiency, states WebMD. A vitamin B-12 deficiency causes nervous system problems, memory loss, weakness and fatigue. Research suggests that vitamin B-12 is safe, even in high doses, as of 2015. People over the age of 14 are recommended to consume 2.4 micrograms of vitamin B-12 daily through a combination of food and supplements. The recommended dosage for children varies by age.