Most food sources of vitamin B12 come from animal products like eggs, milk and meat. If you are a vegan, your doctor may suggest you take a B12 supplement. Physicians also prescribe extra B12 to older people and to patients with illnesses affecting their ability to absorb the vitamin from foods. While overdosing on any supplement is never advised, the risks of taking too much B12 are slim.
Uses
Large amounts of vitamin B12 help prevent pernicious anemia, which occur when your body can't absorb the nutrient from food sources. Older people are prone to this malabsorption issue because of reduced stomach acid, while conditions such as tapeworm, HIV and pancreatic disease also cause difficulty digesting enough of the nutrient. Symptoms of pernicious anemia include fatigue, memory loss, numb feet, mental confusion and weight loss. B12 also helps keep your nervous system functioning properly, and severe deficiencies can lead to serious neurological damage. Extra B12 may also lower the risk of stroke or heart disease for people genetically inclined to high levels of homocysteine, an amino acid which causes health problems when it is elevated.
Overdose Risks
According to the National Institute of Health's Office of Dietary Supplements, taking large amounts of B12 does not carry the risk of toxicity. Known side effects from the supplement, other than possible interactions with medication, are not reported. The body cannot absorb high amounts of the nutrient when it is taken orally, so overdosing from supplements is unlikely, notes the Linus Pauling Institute. Monthly shots as high as 1,000 mcg for pernicious anemia are not uncommon, even though the daily recommended amount of vitamin B12 for the general population of adults is 2.4 mcg.
Drug Interactions
Vitamin B12 supplements may reduce the effectiveness of the antibiotic tetracycline, which treats bacterial infections and acne. If you are taking both tetracycline and any B vitamin or B-complex supplements, take them at different times of day, advises UMMC. Some medications may lower the amount of B12 you absorb from food or supplements. These include chemotherapy drugs and certain medications for diabetes, gout, seizures and excess stomach bile.
B-complex Issues
The University of Maryland Medical Center points out that taking too much of any one B vitamin can diminish the effectiveness of others. An excessive amount of folic acid, or vitamin B9, can lower the amount of B12 in your system, for example. For that reason, people interested in taking extra B12 or any single B vitamin should ask their doctor about B-complex supplements.
Dosage Recommendations
Some vitamins and minerals are so toxic in large doses that the government sets an upper intake level, or UL, for the nutrients. No UL exists for vitamin B12. Teens and adults need about 2.4 mcg of vitamin B12 each day, according to the NIH. The daily recommended amount for toddlers is 0.9 mcg, while children between 4 and 8 need 1.2 mcg and those between 9 and 13 should consume 1.8 mcg of vitamin B12. People with trouble absorbing vitamin B12, including the elderly, may need vitamin supplements. The recommended doses vary depending on the severity of the nutritional deficiency. Food sources of vitamin B12 include fortified cereals, eggs, milk, cheese, yogurt, liver, clams, red meat, chicken, turkey and fish.
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