Vitamin D: What It Does, How Much You Need and What to Buy

What to buy

NatureWise D3 2000 IU is our pick: one small softgel of vitamin D3 a day, on the NSF/ANSI 173 testing list. Of the 19 products we score, 7 are on a testing list. If your doctor wants you on 5,000 IU, or you want K2, a lower price or a vegan spray, see the best vitamin D for your needs.

Where to go next

• Best vitamin D: our pick and the best one for each need.
• Vitamin D dosage guide: how much to take for your age and situation.
• Third-party tested vitamin D: which bottles are on a testing list.
• Vitamin D3 prices: every bottle we track, cheapest per dose first.
• Best vitamin D3 K2 supplement: the D3 + K2 combos compared by testing.
• D2 vs D3, with or without K2, 1,000 vs 5,000 IU and D3 5000 IU with K2.
• Nature Made vs NOW Foods: two big brands side by side.
• Signs you are low and the vitamin D blood test.
• Best time to take it, side effects and the IU to mcg chart.
• Vitamin D for seniors and for women.

What vitamin D does

Your body needs vitamin D to absorb calcium from food, which keeps bones strong. Your skin makes it in sunlight, and a few foods have it. Here is what trials found when people took it as a supplement.

Who runs low on vitamin D?

About 1 in 5 US adults (21%) has a blood level below 50 nmol/L, the level the NIH calls enough for most people, and about 1 in 20 (5.5%) is below 30 nmol/L, which is deficient. That is from our own analysis of the newest national blood data (NHANES 2021–2023).

Younger adults run low most often: 31% of adults 18 to 39 are below 50 nmol/L, against 10% of adults 60 and over. You are also more likely to run low if you have darker skin, carry extra weight, get little sun, or have a condition that blocks fat absorption, like Crohn’s or celiac disease. Breastfed babies need 400 IU a day. A blood test tells you your level; see the vitamin D test.

How long does vitamin D take to work?

Blood levels climb for about two to three months on a steady daily dose, then level off. In a careful winter study, each extra 1,000 IU a day raised blood levels by about 7 ng/mL (Heaney 2003). People who start low rise more, and people with obesity rise less.

How much to take

Adults need 600 IU a day up to age 70 and 800 IU after that. The upper limit for adults is 4,000 IU a day from all sources. Pick vitamin D3, which raises blood levels more than D2, and take it with a meal. The dosage guide has the dose for your situation.

Is vitamin D safe?

Yes, at 1,000 to 2,000 IU a day, for most adults. Too much vitamin D is rare and almost always comes from very large doses or a labeling mistake. It raises calcium in the blood, which causes nausea, thirst and confusion. Ask your doctor or pharmacist first if you:

• Take a thiazide water pill, a steroid like prednisone, orlistat or a statin.
• Have sarcoidosis, kidney disease or high calcium.
• Want to add K2 and take warfarin; see with or without K2.

Questions people ask