Vitamin D is one of the most popular supplements in the world, and one of the most confusing. One friend takes 1,000 IU, another takes 5,000, a podcast host swears by 10,000, and the bottle in your cabinet says 2,000. Who is right? The honest answer is that most adults need less than the internet suggests, a few people need more, and the only way to know where you stand is to look at your own situation.
The official numbers
In the United States, the National Academies set the recommended amounts. The U.S. Office of Dietary Supplements summarizes them on its vitamin D fact sheet:
- Ages 1 to 70: 600 IU (15 mcg) a day
- Over 70: 800 IU (20 mcg) a day
- Babies under 12 months: 400 IU (10 mcg) a day
- Upper limit for adults: 4,000 IU (100 mcg) a day from all sources
Labels now often list vitamin D in micrograms first. To convert, multiply micrograms by 40 to get IU. So 25 mcg is 1,000 IU and 50 mcg is 2,000 IU.
These amounts assume you get little or no sun. They are set to cover nearly everyone in a healthy population, which means many people need even less.
Why so many people take more
Higher doses became popular because vitamin D is hard to get from food, and because many people living far from the equator, or spending most of their day indoors, run low in winter. A daily dose of 1,000 to 2,000 IU is common, sits well below the upper limit, and is a reasonable choice for many adults who rarely see the sun. Doses above 4,000 IU a day should be something your doctor suggests after a blood test, not something you start because of a video.
Who is more likely to run low
- Older adults. Skin makes less vitamin D with age.
- People with darker skin. More melanin means skin makes less vitamin D from the same sunlight.
- People who cover most of their skin or are mostly indoors, including nursing home residents and night workers.
- People with obesity. Vitamin D is stored in body fat, so blood levels tend to be lower.
- People with conditions that affect fat absorption, like Crohn's disease, celiac disease or a history of weight-loss surgery.
- Breastfed babies, since breast milk is low in vitamin D. Pediatricians usually recommend a 400 IU infant drop.
Should you get tested?
The blood test measures 25-hydroxyvitamin D. Here is a rough guide to the results, based on National Academies ranges:
- Below 12 ng/mL (30 nmol/L): deficient
- 12 to 20 ng/mL (30 to 50 nmol/L): may be too low for bone health in some people
- 20 ng/mL (50 nmol/L) or higher: adequate for most people
- Above 50 ng/mL (125 nmol/L): higher than needed, and possibly too high
You may notice that some labs and doctors use 30 ng/mL as the cutoff. Experts genuinely disagree on this, which is part of why the numbers feel confusing.
Routine testing for healthy adults without symptoms is not usually recommended. The Endocrine Society's 2024 guideline advised against routine screening in healthy adults under 75. Testing makes more sense if you are in one of the higher-risk groups above, have bone loss, take medicines that affect vitamin D, or have symptoms your doctor wants to look into. See supplements and lab tests for tips on getting accurate results.
D2 or D3?
Both forms work. Vitamin D3 (cholecalciferol) tends to raise blood levels a bit more and keep them up longer than D2 (ergocalciferol). D3 traditionally comes from sheep's wool, though vegan D3 from lichen is now widely available. D2 is plant-based and often used in prescription doses. Our vitamin D2 vs D3 comparison walks through the details, and the research summary covers what the studies show.
How to take it
- Take it with a meal that has some fat. Vitamin D is fat-soluble and absorbs better that way. See taking supplements with or without food.
- Daily or weekly both work. A daily dose is easiest to remember. Weekly doses are sometimes prescribed, but very large single doses given once or twice a year have not performed well in studies and are best avoided.
- Count all your sources. Add up your multivitamin, calcium-plus-D tablet, fortified milk and any standalone D supplement. It is surprisingly easy to double up.
Can you take too much?
Yes. Vitamin D toxicity is uncommon, but it happens, almost always from supplements rather than sun or food. It causes high blood calcium, which can lead to nausea, vomiting, confusion, extreme thirst, frequent urination and kidney stones. Most cases involve people taking 10,000 IU or more a day for months, or products that contained far more than the label said. That is one reason to choose a product with a USP or NSF seal.
Vitamin D can also interact with some medicines, including certain diuretics and steroids. If you take prescription medicines, check with your pharmacist. Our post on supplement and drug interactions explains how to ask.
What vitamin D will and will not do
Vitamin D helps you absorb calcium and keeps bones strong. Correcting a true deficiency matters, especially for older adults at risk of falls and fractures. Large trials, such as the VITAL study of more than 25,000 adults, found that giving vitamin D to people who were not deficient did not prevent cancer, heart disease or fractures overall. In other words, more is not better once you have enough. Read vitamin D and immune health for a closer look at the immune claims.
The bottom line
For most adults, 600 to 800 IU a day covers the basics, and 1,000 to 2,000 IU is a reasonable, safe choice if you get little sun. Stay at or below 4,000 IU a day unless your doctor tells you otherwise. If you are in a higher-risk group, ask about a blood test. To pick a product, use our guide to comparing vitamin D supplements or browse the vitamin D ingredient page.