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B12 deficiency can cause irreversible nerve damage if untreated. If you have neurological symptoms (tingling, numbness, balance issues), see a doctor promptly. This calculator is for education only.
Calculate your optimal B12 intake based on diet, risk factors, and symptoms
Normal range: 200-900 pg/mL; optimal: 400-800 pg/mL
| Form | Type | Pros | Cons |
|---|---|---|---|
| Cyanocobalamin | Synthetic | Most studied, stable, affordable | Requires conversion to active forms |
| Methylcobalamin | Active coenzyme | Active form, good for nerve health | Less stable, light-sensitive, more expensive |
| Hydroxocobalamin | Natural form | Longer-lasting in body, used in injections | Less common in oral supplements |
| Adenosylcobalamin | Active coenzyme | Active form for energy metabolism | Less common, often combined with methyl |
Bottom line: For most people, cyanocobalamin is fine and well-researched. If you have specific concerns about MTHFR mutations or neurological issues, methylcobalamin may be preferred. The body converts all forms effectively in adequate doses.
| Food | Serving | B12 (mcg) | % Daily Value |
|---|---|---|---|
| Clams | 3 oz (85g) | 84 mcg | 3500% |
| Beef liver | 3 oz (85g) | 70 mcg | 2917% |
| Nutritional yeast (fortified) | 2 Tbsp (16g) | 8.3 mcg | 346% |
| Salmon | 3 oz (85g) | 4.8 mcg | 200% |
| Tuna | 3 oz (85g) | 2.5 mcg | 104% |
| Eggs | 2 large | 1.1 mcg | 46% |
| Milk | 1 cup | 1.2 mcg | 50% |
| Greek yogurt | 6 oz | 1.0 mcg | 42% |
Note for vegans: There are NO reliable plant sources of B12. Seaweed, spirulina, and fermented foods contain B12 analogs that don't work in the human body and may actually worsen deficiency. Fortified foods (nutritional yeast, plant milks, cereals) or supplements are essential.
Absorbs 1.5-2mcg per meal maximum, requires intrinsic factor from stomach. This is why pernicious anemia patients can't absorb B12 normally.
About 1% of a dose absorbs passively. 1000mcg dose = ~10mcg absorbed. This is why high-dose oral B12 works even for pernicious anemia.
Neurological damage from B12 deficiency can become permanent if not treated promptly!
B12 has no established upper limit because excess is excreted in urine. Doses up to 2000mcg daily are common and considered safe. Some people take 5000mcg or more without issues. However, very high doses aren't more effective if you're not deficient—it's just expensive urine.
For most people, high-dose oral B12 (1000-2000mcg) works as well as injections. The 1% passive absorption means you still get 10-20mcg from a 1000mcg pill—way more than the RDA. Shots may be preferred for severe deficiency, pernicious anemia, or if compliance is an issue.
Both work well. Methylcobalamin is "active" but less stable; cyanocobalamin is stable and well-studied but requires conversion. Some people prefer methyl for neurological concerns or due to MTHFR gene variants, but there's no strong evidence it's superior for most people. Price and availability often matter more.
B12 is made by bacteria in soil and animal guts—not by plants. Historically, humans got B12 from drinking untreated water and eating unwashed produce. Modern hygiene removed this source. Animals are now supplemented with B12 too. Vegans aren't "missing" anything—they just need to get their B12 directly from supplements instead of through an animal intermediary.
"Normal" ranges are based on deficiency (below 200 pg/mL causes anemia), not optimal levels. Many experts consider 400-600+ pg/mL optimal. Also check MMA (methylmalonic acid) and homocysteine— these are more sensitive markers that can be elevated even with "normal" B12. If truly replete, look at other causes: iron, thyroid, vitamin D, sleep, stress.
Blood levels can normalize within days to weeks with treatment. Symptoms improve over weeks to months—fatigue often improves first, while neurological symptoms may take 3-12 months to resolve (and may not fully reverse if treatment was delayed). This is why early treatment is crucial.