Most healthy adults need 600 to 800 IU of vitamin D and 2.4 mcg of vitamin B12 per day. Those are the official recommended dietary allowances, but the amount you actually need from a supplement depends on your diet, your age, how much sun you get, and whether you have a deficiency. Here’s how to figure out what makes sense for you.
Daily Vitamin D Recommendations by Age
The National Academies of Sciences, Engineering, and Medicine sets the recommended dietary allowance for vitamin D at 600 IU (15 mcg) per day for everyone between ages 1 and 70, and 800 IU (20 mcg) per day for adults over 70. Infants under 12 months need 400 IU daily.
These numbers assume minimal sun exposure and are designed to maintain blood levels of at least 20 ng/mL, the threshold considered adequate for bone and overall health. Many people fall short of that, especially during winter months, in northern latitudes, or if they have darker skin. If you spend very little time outdoors or consistently wear sunscreen, a daily supplement of 1,000 to 2,000 IU is a common and safe choice for general maintenance.
The tolerable upper intake level for adults is 4,000 IU per day. Going above that without medical guidance increases the risk of pushing blood levels past 50 ng/mL, where adverse effects like excess calcium buildup become a concern. Blood levels above 60 ng/mL are more clearly linked to harm.
Daily Vitamin B12 Recommendations by Age
The RDA for vitamin B12 is 2.4 mcg per day for anyone 14 or older. Pregnant women need 2.6 mcg, and breastfeeding women need 2.8 mcg. Children need less: 0.9 mcg for ages 1 to 3, 1.2 mcg for ages 4 to 8, and 1.8 mcg for ages 9 to 13.
Unlike vitamin D, there is no established upper limit for B12. Both the U.S. Food and Nutrition Board and European regulators have concluded there isn’t enough evidence of harm from high doses to set one. Your body excretes what it doesn’t need through urine, so taking more than the RDA is generally not dangerous for healthy people.
When You Might Need More Vitamin D
If a blood test shows your levels are below 20 ng/mL, you’re considered deficient. The Endocrine Society’s clinical guideline recommends a loading phase of 50,000 IU once a week for eight weeks (or roughly 6,000 IU daily) to bring levels back up, followed by a maintenance dose of 1,500 to 2,000 IU per day.
People who are obese or have conditions affecting fat absorption often need two to three times more vitamin D than the standard correction dose, sometimes 6,000 to 10,000 IU daily, because vitamin D is fat-soluble and gets sequestered in body fat. Certain medications, including some anti-seizure drugs and steroids, also interfere with vitamin D metabolism and may require higher doses. These higher-dose protocols should be guided by blood testing.
When You Might Need More B12
B12 deficiency shows up on a blood test as levels below 200 pg/mL. The borderline range, 200 to 300 pg/mL, often warrants further testing. Levels above 300 pg/mL are considered normal.
Several groups are at higher risk of running low. Vegans and strict vegetarians are the most obvious, since B12 occurs naturally only in animal products. Studies consistently find that 50 to 70% of vegans in various countries have suboptimal B12 status. A daily supplement of 50 to 100 mcg is commonly recommended for vegetarians and vegans, well above the 2.4 mcg RDA, because only a fraction of an oral dose is absorbed at any one time. A minimum of 4 mcg per day is associated with adequate status in adults who eat little or no animal food, but most experts recommend a larger buffer.
Adults over 50 are another at-risk group. With age, the stomach produces less acid, making it harder to extract B12 from food. Supplemental B12 doesn’t require stomach acid for absorption, which is why guidelines specifically recommend that older adults get their B12 from fortified foods or supplements rather than relying on meat and dairy alone.
Several common medications also reduce B12 absorption. Acid-blocking drugs (omeprazole, lansoprazole, pantoprazole, famotidine, and similar medications) lower stomach acid and can deplete B12 over time. Metformin, widely prescribed for type 2 diabetes, is another well-known culprit. If you take any of these long-term, periodic B12 monitoring is worthwhile.
Injections vs. Oral Supplements for B12
When deficiency is severe or caused by a condition that prevents absorption in the gut, such as pernicious anemia, injections have traditionally been the go-to treatment. A typical injection protocol starts with 1 mg daily or every other day for a week or two, tapers to weekly, then settles into monthly injections indefinitely.
Oral supplements can work even for people with absorption problems, though. A small percentage of B12 is absorbed passively through the intestinal wall regardless of the usual absorption pathway, so high oral doses (1,000 mcg per day) can compensate. Clinical guidelines in several countries now list high-dose oral B12 as an acceptable alternative to injections for most patients.
Which Supplement Form to Choose
Vitamin D supplements come as D2 (ergocalciferol) and D3 (cholecalciferol). D3 is generally preferred because it raises and maintains blood levels more effectively, but D2 works too, especially at the higher doses used for deficiency correction. Vitamin D is fat-soluble, so taking it with a meal that contains some fat improves absorption.
Magnesium plays a role in converting vitamin D to its active form in the body, so being low in magnesium can blunt the benefit of vitamin D supplements. Vitamin K2 also works alongside vitamin D in directing calcium into bones rather than soft tissues. Neither is strictly “required” to take with vitamin D, but ensuring you’re not deficient in magnesium is a practical step, especially since magnesium deficiency is common and often goes unnoticed.
For B12, the two main supplement forms are cyanocobalamin and methylcobalamin. Cyanocobalamin is the most studied and typically the cheapest. In a study of vegans, those taking cyanocobalamin maintained significantly higher blood markers of active B12 (a median of 150 pmol/L) compared to those taking methylcobalamin (median of 78.5 pmol/L). Absorption rates for a single microgram are similar, around 44 to 49%, but cyanocobalamin appears to perform better at higher doses. Methylcobalamin may be retained slightly better since less of it is excreted in urine, but the overall evidence favors cyanocobalamin for reliably maintaining B12 status.
Practical Starting Points
If you eat a varied diet, get some sun, and have no diagnosed deficiency, a daily supplement of 1,000 to 2,000 IU of vitamin D3 and the RDA of 2.4 mcg of B12 (easily covered by most multivitamins) is a reasonable baseline. If you’re vegan, bump B12 to 50 to 100 mcg daily or take 1,000 mcg two to three times per week. If you’re over 50, a standalone B12 supplement in the range of 25 to 100 mcg daily provides a reliable safety net.
For vitamin D, a blood test is the only way to know your actual level and whether your current intake is enough. This is especially true if you have risk factors for deficiency: limited sun exposure, darker skin, obesity, or a digestive condition that impairs fat absorption. B12 blood testing is similarly straightforward and worth doing if you fall into a higher-risk category or notice symptoms like unusual fatigue, numbness or tingling in your hands and feet, or difficulty with balance and memory.