How to Choose Vitamin D: Form, Dosage, and iHerb Options

Vitamin D is essential to the body: it participates in calcium and phosphorus metabolism and supports healthy bones and the normal function of the muscles, nervous system, and immune system. But it is fat-soluble, so the idea that “more is better” is especially inappropriate here.

In brief: For most shoppers, vitamin D3 (cholecalciferol) in a dose appropriate for their age and needs is the most straightforward choice. An oil-based form is convenient, but it is not an absolute requirement for absorption. High doses should not be taken continuously without a medical reason.

Contents

• What is vitamin D?
• Vitamin D2 or D3: which should you choose?
• Does vitamin D have to be oil-based?
• What vitamin D dosage is needed?
• Does everyone need a vitamin D blood test?
• What 25(OH)D level is considered normal?
• Why should you avoid taking high doses on your own?
• Do you need vitamin D3 with K2?
• How should you choose vitamin D for a child?
• Which vitamin D should you choose on iHerb?
• How should you take vitamin D?
• Summary

What is vitamin D and why is it needed?

Vitamin D is a fat-soluble vitamin that the body obtains from several sources:

• it is produced in the skin under exposure to ultraviolet light;
• it comes from food;
• it can come from fortified foods and dietary supplements.

One of vitamin D’s best-established functions is helping the body absorb calcium and maintain bone health.

Vitamin D also participates in the function of the muscles, nervous system, and immune system.

Severe deficiency can cause rickets in children and osteomalacia in adults—a disorder involving inadequate mineralization of bone tissue.

However, it is not correct to attribute almost any feeling of ill health, frequent colds, fatigue, or other nonspecific symptoms specifically to low vitamin D.

Vitamin D2 or D3—which should you choose?

Supplements generally contain two main forms:

• vitamin D2—ergocalciferol;
• vitamin D3—cholecalciferol.

Both forms can raise vitamin D levels in the body, and both are absorbed in the intestine.

However, research indicates that D3 is generally more effective than D2 at maintaining the blood concentration of 25(OH)D, particularly with longer-term use.

For a standard dietary supplement, I would therefore usually choose:

Vitamin D3 / Cholecalciferol.

If you follow a vegan diet, check the source of D3. Cholecalciferol has traditionally been obtained from lanolin, but plant-based versions made from lichen are also available.

Does vitamin D have to be oil-based?

Older recommendations often stated that vitamin D must be dissolved in oil or it would barely be absorbed.

That is too categorical.

Vitamin D is fat-soluble, and fat in the intestine can improve its absorption. But vitamin D can still be absorbed without consuming a large amount of fat at the same time.

You can therefore choose:

• oil-based softgels;
• liquid drops based on MCT or another oil;
• tablets;
• dry capsules;
• chewable forms.

What matters more than the carrier is:

• dosage;
• ease of use;
• the ingredient list;
• age suitability;
• consistent use.

If you take vitamin D as a tablet or dry capsule, taking it with a regular meal that contains some fat is reasonable.

What vitamin D dosage is needed?

This is an area where older articles especially need correction.

Doses of 2,000–4,000 IU per day should not automatically be called a preventive standard for every adult.

For example, the recommended daily amounts established by the National Academies and listed by the NIH are:

Age or life stage	Recommended amount
0–12 months	400 IU (10 mcg)
1–18 years	600 IU (15 mcg)
19–70 years	600 IU (15 mcg)
71 years and older	800 IU (20 mcg)
Pregnancy and breastfeeding	600 IU (15 mcg)

These figures refer to total daily vitamin D intake from food and supplements in healthy people.

Important: Treatment for confirmed deficiency is a separate issue. It may involve a dose considerably higher than the usual daily amount and should be selected individually by a medical professional.

In 2024, the Endocrine Society separately emphasized that healthy adults under age 75 generally should not take vitamin D above standard recommended amounts simply to “prevent disease.”

Does everyone need a vitamin D blood test?

No.

This is another important change from commonly repeated advice in previous years.

The main test is called:

25(OH)D—25-hydroxyvitamin D.

However, current recommendations do not support routine screening of all healthy people without symptoms or risk factors.

In its 2024 recommendations, the Endocrine Society advises against regular 25(OH)D testing in healthy adults solely to decide whether they should take vitamin D.

The USPSTF also finds that there is not enough evidence to recommend mass screening of asymptomatic adults.

Testing may nevertheless be appropriate in certain diseases, calcium-metabolism disorders, bone conditions, malabsorption, after some surgeries, and other clinical situations.

In other words, “everyone must test first and then take vitamin D” is also too simplistic today.

What vitamin D level is considered normal?

There is no universal “ideal value” here either.

In the past, people were often advised to aim for levels of 50–80 ng/mL. There is now no evidence-based justification for using that range as a universal target.

According to the NIH:

25(OH)D	Interpretation
Less than 12 ng/mL	Associated with deficiency
12–20 ng/mL	May be inadequate for bone health in some people
20 ng/mL or higher	Generally sufficient for bone health in most healthy people
Above 50 ng/mL	May be associated with adverse effects

Professional organizations also emphasize that a universal “optimal” 25(OH)D level for preventing all diseases has not been established.

Why should you avoid taking 5,000–10,000 IU of vitamin D on your own?

On iHerb, it is easy to find vitamin D in doses of:

• 1,000 IU;
• 2,000 IU;
• 5,000 IU;
• 10,000 IU;
• and even 50,000 IU.

The fact that a dose is sold does not mean it is intended for daily self-use.

For adults, the tolerable upper intake level for long-term daily vitamin D intake from all sources is:

4,000 IU (100 mcg) per day.

This is not a recommended daily dose; it is an upper limit above which the risk of adverse effects increases.

When treating confirmed deficiency, a doctor may temporarily prescribe higher doses, but that is therapy—not routine prevention.

Excessive vitamin D intake can raise blood calcium and lead to nausea, weakness, dehydration, kidney stones, impaired kidney function, and other serious complications.

Should you increase vitamin D when you have a cold?

I would remove the advice from the older article suggesting that vitamin D dosage should be increased during an acute respiratory infection.

Vitamin D is connected with immune function, but that does not mean you should increase the dose on your own every time you catch a cold.

High doses of vitamin D are not a universal treatment for acute respiratory infections.

If you already take vitamin D at a dose appropriate for you, do not change it just because cold symptoms appear unless a doctor advises you to do so.

Do you need to choose vitamin D3 with K2?

These combinations are very popular on iHerb:

Vitamin D3 + K2.

The logic is understandable: both vitamins are involved in calcium metabolism and bone health.

But that does not mean everyone taking vitamin D automatically needs additional K2.

If you do not have a separate reason to take vitamin K, ordinary vitamin D3 remains a reasonable choice.

Especially important: Vitamin K can interact seriously with warfarin and some other anticoagulants. If you take these medicines, do not choose D3 + K2 products without consulting a doctor.

How should you choose vitamin D for a child?

Liquid drops are especially convenient for infants.

For the first year of life, the standard recommended amount is 400 IU (10 mcg) of vitamin D per day.

For breastfed or partially breastfed infants, the American Academy of Pediatrics recommends 400 IU of vitamin D daily, beginning in the first days of life.

When choosing children’s drops, pay less attention to the bottle size than to how much vitamin D is in one drop.

This is essential: with one manufacturer, 400 IU may be in a single drop; with another, it may be in several drops or in 1 mL of solution.

Errors when converting the concentration can lead to overdose.

Which vitamin D should you choose on iHerb?

I would not create a ranking of the “best” vitamin D. It is more useful to choose a product for a specific purpose.

1. Vitamin D for infants—400 IU per drop

California Gold Nutrition, Baby Vitamin D3 Drops, 400 IU

Each drop contains 400 IU (10 mcg) of D3. The base is MCT from coconut oil.

This format is convenient because one drop corresponds to the standard daily amount for an infant. The dose for a particular child should be discussed with a pediatrician.

2. Liquid vitamin D3—1,000 IU

California Gold Nutrition, Vitamin D3, 1,000 IU

This is a liquid MCT-based option. A serving of two drops contains 1,000 IU of vitamin D3.

It is convenient for people who do not like capsules and prefer a liquid format.

3. NOW Foods Vitamin D3—2,000 IU

NOW Foods, Vitamin D3, 2,000 IU, 120 softgels

One of the most popular vitamin D options on iHerb. Each softgel contains 2,000 IU of cholecalciferol.

Important: 2,000 IU is already above the standard RDA for most healthy adults, so I would not automatically call this dose necessary for everyone.

4. Vitamin D3 + K2

Sports Research, D3 + K2, 2,500 IU / 100 mcg

Each softgel contains 2,500 IU of plant-based vitamin D3 from lichen and 100 mcg of vitamin K2.

This option is worth considering only if you actually need a D + K combination. If you take warfarin or similar anticoagulants, products containing vitamin K require consultation with a doctor.

How I choose vitamin D on iHerb:

• D3 rather than D2 unless there is a specific reason to choose D2;
• a clear dose per capsule or drop;
• the minimum necessary dose rather than the “strongest” option;
• a convenient format that you can realistically take consistently;
• no unnecessary additional ingredients;
• for children, especially careful attention to the concentration per drop.

How should you take vitamin D?

Vitamin D can be taken with food.

Because it is fat-soluble, taking it with a regular meal containing fat may improve absorption.

There is no need to drink a spoonful of oil specifically with vitamin D.

In most cases, consistency matters more than the time of day.

If a doctor has prescribed a specific treatment plan for confirmed deficiency, follow that plan rather than the general instructions in an article or iHerb reviews.

Who should be especially careful to discuss vitamin D with a doctor?

Extra caution is needed if you have:

• kidney disease;
• calcium-metabolism disorders;
• conditions involving impaired fat absorption;
• a history of stomach or intestinal surgery;
• use of thiazide diuretics;
• use of orlistat;
• long-term use of certain glucocorticoids;
• other diseases or medicines that may affect vitamin D metabolism.

In these situations, the standard approach of “buy 2,000 IU and take it every day” may not be appropriate.

Which vitamin D should you choose: summary

Choosing vitamin D is actually simpler than it may seem after browsing hundreds of products on iHerb.

The essentials:

• for most supplements, vitamin D3 (cholecalciferol) is a reasonable choice;
• an oil-based form is convenient but not an absolute requirement for normal absorption;
• for infants, the standard recommendation is 400 IU per day;
• for healthy adults, the standard RDA is considerably lower than the popular 2,000–5,000 IU doses;
• not every healthy person needs regular 25(OH)D testing;
• there is no established need for everyone to aim for a level of 50–80 ng/mL;
• 5,000–10,000 IU should not be chosen simply because that dose is sold on iHerb;
• K2 is not a mandatory addition to every vitamin D supplement;
• with confirmed deficiency, the dose is determined separately.

I would therefore choose vitamin D not by asking “which one is strongest?” but by asking which form and dose actually match your needs.

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Important: This article is for informational purposes and does not replace medical advice. If you have confirmed vitamin D deficiency, a medical condition, are pregnant, take medicines, or are choosing a dose for a child, discuss supplementation with a medical professional.