How long does vitamin D take to work?

The trials we cite measured blood vitamin D over weeks to months, not how anyone felt. In 97 adults on about 800 IU a day, blood levels were up 32% at three months and 50% at six. No trial we cite measured a feeling, or a health outcome in healthy adults, within weeks.

What "work" can mean, and what was measured

There are three different questions inside this search, and they have different answers.

• How long until my blood level changes? Weeks to months. This is the thing trials measured.
• How long until I feel something? No trial we cite measured a feeling at any time point, so we cannot give you a number, and we will not say you will notice anything.
• How long until it changes a health outcome? The large trials ran for years, and in generally healthy adults at 2,000 IU a day they found no difference in the outcomes they measured. Details below.

The blood level: weeks to months

Vitamin D that you swallow is converted in the body to 25-hydroxyvitamin D, the form measured in a blood test. A review of how vitamin D behaves in the body gives that circulating form a half-life of about 15 days, and the vitamin itself, held in fat, about two months. As arithmetic, not a trial result: a substance with a 15-day half-life takes roughly five half-lives, about 75 days, to settle close to a new steady level. That fits what the trials did.

• 20 weeks, daily doses. In a controlled dosing study in 67 men in Omaha over winter, daily vitamin D3 at 25, 125 and 250 mcg (1,000, 5,000 and 10,000 IU) or none was given for about 20 weeks, with blood levels measured at intervals. The settled level rose in direct proportion to the dose, about 0.70 nmol/L for each additional microgram a day, from a starting average of 70.3 nmol/L. The abstract does not state the week at which levels levelled off, and it did not use our dose.
• 3 and 6 months, about 800 IU a day. In a randomised Swiss trial of 97 adults whose levels started below 50 nmol/L, weekly or monthly doses equal to 800 IU a day raised the level by 32% after 3 months and 50% after 6 months. Only about a third reached 75 nmol/L on that dose, so how far and how fast it moves varies from person to person.

So "what to expect", from the durations the trials used, is: a blood test after about 3 months would show a settled level; a test after a week or two would not. Neither trial used 2,000 IU a day, which is the amount in our softgel, as low as $15.35 a bottle on subscription, so we make no prediction about a number it will reach.

Health outcomes: years, and null at 2,000 IU

The trials that tested health outcomes used the same dose as our softgel and ran for years. In VITAL, 25,871 US adults (men 50 and over, women 55 and over) took 2,000 IU a day of vitamin D3 or placebo for a median of 5.3 years. There was no lower rate of invasive cancer (hazard ratio 0.96, 95% CI 0.88 to 1.06) or of major cardiovascular events (0.97, 0.85 to 1.12). A companion analysis in adults aged 50 and over found no difference in depression risk or depressive symptoms (0.97, 0.87 to 1.09). People in these trials were not selected for low vitamin D, so they do not tell you what happens in someone who is low. We make no claim that taking it changes any of those outcomes.

How long should you take vitamin D3?

No trial we cite defines a right duration. VITAL ran for a median of 5.3 years at 2,000 IU a day with no excess of adverse events identified, which tells you about safety at that dose over that time, not that you must continue. The question of whether you need it at all is one for a blood test and a clinician. If you stop, expect the level to fall gradually, not suddenly: with a 15-day half-life for the circulating form and about two months for stored vitamin D, it declines over weeks to months. We cite no trial of stopping, so we cannot say what you would notice.

Does it work, then?

If "work" means raising the blood level, daily vitamin D does, in the trials above, on a timescale of months. If it means preventing or treating something, our dose has been tested at scale in generally healthy adults, and the results above were null. If you are unsure which applies to you, the amount and timing are covered in how much vitamin D should you take a day and when to take it, and what the research does and does not show in vitamin D benefits.

Questions people ask

How long does vitamin D3 take to work, and how quickly does it work?

Blood levels: months. In the trials we cite, a settled level took roughly 3 months or more, and the 20-week study tracked it over that time. We found no data on how soon anyone feels different.

Does a vitamin D3 supplement work?

It raises the blood level of vitamin D, in proportion to the dose. At 2,000 IU a day in healthy older adults, the large trials found no effect on cancer, cardiovascular events or depression.

How long should you take vitamin D3?

No trial defines it. The longest at our dose ran a median of 5.3 years. Ask a clinician, ideally with a blood test.