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Vitamin A is a potent, immune-enhancing nutrient and megadoses are sometimes suggested by integrative doctors at the onset of certain viral illnesses. This article covers doses for all ages, notes about pregnancy and lactation, and cautions to prevent overdose. 

What Is Vitamin A?

Vitamin A is a fat-soluble vitamin that is important for growth, reproduction, vision, and immunity.

It is available in our diet in two forms:

  1. Preformed vitamin A (retinol) – present in many animal products
  2. Provitamin A (beta carotene) – found in leafy greens and orange/yellow fruits and vegetables

Read more: Top Food Sources of Vitamin A – Dr. Green Mom

How Does Vitamin A Affect The Immune System?

Vitamin A has multiple positive effects on immune function:

  1. Vitamin A strengthens the mucosal barriers of the mouth, intestines, and urogenital tract. These barriers are our important first line of defense against disease-causing germs. 
  2. Vitamin A regulates important immune tissues: the thymus and bone marrow.
  3. Vitamin A is needed for proper function of immune cells including T cells, B cells, neutrophils, and macrophages. These are the cells that identify and eliminate infection. 
  4. Vitamin A plays a role in maintaining lung health during infectious diseases, especially Covid-19. 

Vitamin A Depletion During Infection

During some infections, most notably Covid-19 and measles, vitamin A levels go down through decreased appetite, less intestinal absorption (especially if diarrhea is present), and increased excretion in urine. This means that during some infections, particularly viral respiratory and digestive infections, Vitamin A requirements go up just as Vitamin A levels go down. Therefore, vitamin A intake, either through food or supplements, should be a priority when illness strikes. 

Megadoses of vitamin A are one possible strategy for maintaining vitamin A levels during illness, but this isn’t appropriate for all people. See the section below for more details. 

When & How To Give A Megadose Of Vitamin A

Vitamin A megadoses are sometimes given at the onset of certain viral illnesses, especially measles, RSV, and Covid-19.

As a megadose, most sources online recommend quite high doses of up to 400,000 IU given over two days (200,000 IU per day). In general, this dose is well tolerated according to scientific literature.

The above dose is based on what has been used in economically disadvantaged nations where nutrient deficiencies are rampant and much more severe than what is seen in the United States. In Japan, a country where malnutrition isn’t common, a study was conducted with a single dose of 100,000 IU of vitamin A for children and infants with measles and RSV. This study showed a significant benefit to these well-nourished children and no side effects were noted. The ages in this study were 1 month – 4.5 years. 

In my practice, I recommend megadoses similar to what was examined in the Japan study. However, in some cases, where there is a risk of vitamin A deficiency, I recommend the larger standard megadose. I also usually recommend the larger dose at the onset of measles or Covid-19. Though megadoses of vitamin A have been safely given to very young infants, including preterm infants, in my own clinical practice, I usually prefer to wait until babies are at least 6 months old if possible. A discussion with your own doctor about the risks and benefits is a must.  

Read more: 5 Signs of Vitamin A Deficiency – Dr. Green Mom

Vitamin A Megadose Chart

Age Conventional Medicine Vitamin A Megadose Dr. Green Mom’s Vitamin A Megadose (½ of conventional) Single Megadose of Vitamin A From Japan Study
0-6 months 50,000 IU x 2 days 25,000 IU x 1 or 2 days 100,000 IU x 1 day
6-12 months 100,000 IU x 2 days 50,000 IU x 1 or 2 days 100,000 IU x 1 day
12 months and over 200,000 IU x 2 days 100,000 IU x 1 or 2 days 100,000 IU x 1 day

Vitamin A Megadose VS. Daily Dosing

In some cases, my patients have already been supplementing with Vitamin A drops, cod liver oil, or beef liver capsules. If such patients get a viral disease, I usually have them continue their vitamin A protocol and skip the megadose. 

Vitamin A Megadose Frequency

In cases when a patient has had a recent megadose of vitamin A (within 3 months), I do not recommend repeating the high-dose vitamin A regimen. To prevent vitamin A toxicity, I generally limit vitamin A megadoses to two per year at least 3 months apart.   

For more information about vitamin A: Benefits of Vitamin A + Pediatric Dosing – Dr. Green Mom

Vitamin A Supplementation Notes For Pregnancy 

Pregnant women should not take large doses of vitamin A. This can potentially cause pregnancy loss or birth defects. Instead, I usually have pregnant women maintain their vitamin A levels through a combination of diet and prenatal multivitamins. When a mother in my practice has a difficult time consuming animal products, I may have her supplement with cod liver oil and/or beef liver capsules in addition to her prenatal multivitamin to ensure that she is getting adequate vitamin A. 

The recommended amount of vitamin A during pregnancy is 770 mcg (2500 IU) and the upper limit for vitamin A in pregnancy is 3,000 mcg or 10,000 IU. Because of the potential toxicity of vitamin A during pregnancy, I usually have pregnant patients in my practice aim for between 2500-5000 IU per day of vitamin A from both food and supplements. 

Read more: Vitamin A Supplementation For Moms – Dr. Green Mom

Vitamin A Supplementation Notes For Lactation

Vitamin A supplementation increases the vitamin A concentration of breast milk in vitamin A deficient women, which may have positive health effects for her baby. 

However, if a woman has enough vitamin A, further supplementation won’t cause vitamin A levels in her breast milk to increase much. Evidence suggests that a lactating woman can likely take a single megadose of up to 120,000 mcg (400,000 IU) without passing any potential vitamin A toxicity on to her child. 

Read more: Vitamin A Supplementation For Moms – Dr. Green Mom

Vitamin A Overdose Signs

Vitamin A (retinol) can cause side effects if taken in excess. If a single large dose of vitamin A is consumed (100 times or more the RDA), then toxicity symptoms like aching muscles, blurred vision, severe headache, nausea, and coordination problems may result. Rarely, drowsiness, coma, and even death can occur if the cerebral spinal fluid pressure rises too high.

If vitamin A overdose happens slowly over time, signs and symptoms will gradually appear and worsen including dry skin, muscle pain, joint pain, tiredness, depression, and abnormal liver function tests. 


Vitamin A megadosing is a strategy that may be used to maintain adequate vitamin A levels during viral infections. Having a strong supply of vitamin A is important for maintaining a healthy immune response. However, vitamin A is a fat soluble vitamin that can cause toxicity if taken in excess. Always consult a healthcare provider before taking large doses of vitamin A or administering them to your child. 

For more information about foods high in vitamin A, see this article: Top Food Sources Of Vitamin A – Dr. Green Mom


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Vitamin A – Drugs and Lactation Database (LactMed) – NCBI Bookshelf 

Vitamin A and Carotenoids – Health Professional Fact Sheet 

Dr. Green Mom

Dr. Mayer is a naturopathic medical doctor and an expert in nutrition and wellness as it relates to pediatrics and families. Her passion for prevention of disease as cure fueled her desire to immerse herself into specializing in adult onset chronic conditions, as well as childhood chronic illness.

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