You’re tired. Not just "stayed up too late watching Netflix" tired, but that deep, bone-weary exhaustion that hits at 2 PM and makes you want to curl up under your desk. You’ve googled it. Everyone says it’s B12. So you go to the store, look at the shelf, and see bottles ranging from 5 mcg to 5,000 mcg. It’s chaotic. Honestly, the supplement aisle is a mess of marketing and "more is better" logic that doesn't always hold up to the science.
When people ask vitamin b12 how much should you take, they usually want a single number. But biology is rarely that polite. Your needs change based on whether you're a 22-year-old vegan, a 65-year-old with a picky stomach, or someone taking metformin for diabetes.
The baseline: What the official guidelines say
The National Institutes of Health (NIH) has a standard answer. For most adults, the Recommended Dietary Allowance (RDA) is 2.4 micrograms (mcg) per day. That is a tiny amount. To put it in perspective, a single microgram is one-millionth of a gram. You could fit your entire daily requirement on the tip of a needle and still have room for dessert.
If you're pregnant, that number bumps up to 2.6 mcg. If you're breastfeeding, it goes to 2.8 mcg. This is the bare minimum to keep your nerves from misfiring and your red blood cells from turning into "megaloblasts"—giant, clunky cells that can't carry oxygen worth a lick.
But here’s the kicker: the RDA assumes you have perfect digestion. Most of us don't.
Why absorption is the real bottleneck
Vitamin B12 is a diva. It doesn't just slide into your bloodstream like Vitamin C. It requires a specific protein made in your stomach called Intrinsic Factor (IF). Think of IF as a VIP pass. Without it, the B12 just passes right through you.
This is why "how much you take" is often less important than "how much you actually absorb." If you take a 500 mcg pill, your body might only grab about 10 mcg of it. The absorption mechanism gets saturated very quickly. Once those Intrinsic Factor receptors are full, the rest of that expensive supplement is just making for very pricey urine.
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Vitamin B12 how much should you take if you’re plant-based?
If you don't eat meat, fish, or dairy, you aren't getting B12 from your food. Period. There are myths about fermented soy or spirulina having B12, but those are usually "analogs"—compounds that look like B12 but don't actually function in the human body. They might even block real B12 from working.
For vegans and strict vegetarians, the dose needs to be higher than the RDA because you're likely taking it in one go rather than small doses throughout the day.
- Daily option: A chewable or sublingual tablet of 25-100 mcg.
- Weekly option: A massive dose of 2,000 mcg once a week.
Why the jump to 2,000 mcg? Because when you flood the system with a huge amount, about 1% of it manages to diffuse across the gut membrane even without the help of Intrinsic Factor. It’s a brute-force method that works.
The age factor and the "Stomach Acid" problem
Once you hit 50, things change. The lining of your stomach starts to thin out, a condition called atrophic gastritis. This reduces the amount of acid you produce. You need that acid to unbind B12 from the protein in your steak or eggs.
The CDC and the Institute of Medicine actually recommend that people over 50 get most of their B12 from supplements or fortified foods. Why? Because in those forms, the B12 is already "free" and doesn't need stomach acid to break it loose.
If you’re over 50, a daily supplement of 100 to 400 mcg is often what doctors like Dr. Michael Greger or experts at the Mayo Clinic suggest. It’s insurance. It keeps your brain from shrinking—literally—as B12 deficiency is linked to accelerated brain atrophy in older adults.
Dealing with a real deficiency
If a blood test shows your levels are "in the basement" (usually below 200 pg/mL, though many functional medicine practitioners prefer seeing you above 500 pg/mL), the RDA is useless. You’re in a hole and you need to climb out.
Doctors often prescribe:
- Injections: Usually 1,000 mcg of cyanocobalamin or methylcobalamin injected into the muscle. This bypasses the gut entirely.
- High-dose oral: 1,000 to 2,000 mcg daily for a month or two.
Research, including a well-known Cochrane review, has shown that high-dose oral B12 can be just as effective as shots for many people, provided they actually take it every single day.
Can you take too much?
Basically, no.
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B12 is water-soluble. Your kidneys are incredibly efficient at filtering out the excess. There is no "Tolerable Upper Intake Level" (UL) set for B12 because there’s no evidence of toxicity. That said, some people report acne breakouts when taking massive 5,000 mcg doses. If your skin starts acting like you’re 16 again, maybe dial back the dosage.
There's also some emerging research regarding high B12 levels and lung cancer risk in smokers, or issues for people with chronic kidney disease. If you fall into those camps, definitely talk to a hematologist before going rogue with high doses.
Methylcobalamin vs. Cyanocobalamin: The Great Debate
You’ll see two main types on the label.
Cyanocobalamin is the synthetic version. It’s stable, cheap, and has the most research behind it. It contains a tiny molecule of cyanide (don't panic, it's less than what you’d find in an apple seed).
Methylcobalamin is the "active" form found in nature. People with the MTHFR gene mutation often swear by this version, believing their bodies can't convert the synthetic stuff. While the science is still a bit split on whether it truly matters for the average person, methylcobalamin is generally better retained in the tissues.
If you’re a smoker, stay away from cyanocobalamin. Your body is already dealing with cyanide from cigarettes; it doesn't need the extra work of detoxifying the supplement. Go for the methyl version.
Actionable steps for your B12 routine
Don't just guess. Here is how you actually handle this:
1. Get a lab test. Ask for a "Serum B12" test, but also ask for an uMMA (Urinary Methylmalonic Acid) test. The uMMA is more sensitive. It’s like a "check engine" light that turns on before your B12 levels hit rock bottom.
2. Check your medications. If you take Metformin (for diabetes) or PPIs like Prilosec or Nexium (for acid reflux), you are at a high risk for deficiency. These drugs actively block B12 absorption. You will likely need at least 1,000 mcg daily to compensate.
3. Pick your delivery method. Sublingual (under the tongue) tablets or sprays are great if you have digestive issues. They aren't "magical," but they give the B12 a head start through the mucous membranes in your mouth.
4. Consistency over quantity. Taking 50 mcg every day is much better for your energy levels than taking 5,000 mcg once a month and forgetting the rest of the time.
5. Read the labels of your "milks." Many almond, oat, and soy milks are fortified with B12. If you drink two glasses a day, you might already be hitting your 2.4 mcg RDA.
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The goal isn't to have the highest B12 levels in the world. It's to ensure your brain, nerves, and blood have exactly what they need to keep you feeling human. Start with the lower end of the supplement range (around 100-250 mcg) unless a doctor tells you that you're severely deficient. Monitor how you feel. If the brain fog lifts and the 2 PM slump disappears, you’ve found your "just right" dose.