Vitamin C has many important jobs in your body, but it’s probably best known for supporting your immune system.

Some people wonder if vitamin C is helpful for allergies, which are immune reactions to substances in your environment (1).

This article takes an evidence-based look at vitamin C’s effectiveness, uses, and any precautions for treating or preventing allergies.

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There’s some evidence that vitamin C, also known as ascorbic acid, may help with some allergies.

Vitamin C acts as a natural antihistamine and antioxidant. Studies have shown it may decrease inflammation, swelling, and related symptoms that happen at the site of an allergic reaction (1, 2).

Allergy symptoms happen when your immune system reacts to a foreign invader, called an allergen.

Common allergens include pollen, pet dander, and proteins in certain foods. Cells in your immune system called mast cells are activated and release histamine to help block the invader.

Histamine may trigger the following allergy symptoms (3):

  • runny nose
  • sneezing
  • red, watery eyes
  • itching
  • rash
  • asthma
  • vomiting or diarrhea
  • swelling
  • anaphylaxis, a rare but life threatening swelling in the airway

For mild seasonal or environmental allergies, antihistamine medications can block histamine and its effects, but they can have unwanted side effects of their own (4).

Vitamin C acts differently from antihistamine medications, reducing the amount of antihistamine you produce rather than blocking antihistamine receptors. Research suggests histamine levels may reduce by about 38% after a person takes 2 grams of vitamin C (3, 4, 5).

Receiving a higher dose of vitamin C through an IV might be more effective.

A small study in 89 people with allergies or infectious diseases showed that those who received a 7.5 gram IV (intravenous) infusion of vitamin C had about 50% less histamine in their blood (6).

The study found that people who had allergies benefited from a greater reduction in histamine than those with infectious diseases (6).

Another observational study looked at the effects of giving an intravenous (IV) infusion of vitamin C to people with allergic symptoms in the skin or respiratory system.

It found that a 7.5-gram dose via IV was associated with a reduction in allergy symptoms like runny nose, sneezing, itching, restlessness, and sleep problems in 97% of people with allergies. Only 1 person out of 71 reported side effects (7).

A high quality study also tested a vitamin C nasal spray in 60 people with allergy symptoms, including sneezing and runny nose. The study found it improved symptoms by 74% (8).

The body of research looking at the effects of vitamin C on allergies is relatively small. Scientists need to do more high quality studies in humans to investigate this further.

Summary

Allergy symptoms happen when your body releases histamine in response to an allergen. Vitamin C is a natural antihistamine that several small studies have shown may reduce allergy symptoms.

Vitamin C seems to reduce upper respiratory symptoms that seasonal or environmental allergies cause. Common allergens may include pollen, mold, dust, and pet dander (2, 3, 9).

With these allergens, the histamine reaction occurs in the nose or sinuses, resulting in allergic rhinitis — runny nose, sneezing, congestion, and red, watery eyes. The allergens can also trigger a reaction in your lungs that can lead to asthma (2, 3, 9, 10).

Vitamin C’s antihistamine properties might help reduce allergic rhinitis and asthma, as blood vessels in your respiratory tract have high concentrations of mast cells, which produce histamine (9).

Some research also suggests that the antioxidant properties of vitamin C may protect lung function by protecting cells in your lungs from oxidative damage (11).

Still, there’s no evidence that vitamin C can prevent seasonal or environmental allergies.

Compared with seasonal or environmental allergies, food allergies tend to cause a more serious reaction and can affect your digestive tract, skin, eyes, and throat, as well as your respiratory tract.

In severe cases, a food allergy can cause a deadly anaphylactic reaction in someone who is allergic and has been exposed to the allergen, even if only to just a tiny amount.

There’s no evidence that vitamin C can prevent or treat a food allergy. If you or someone in your family has been diagnosed with a food allergy, it’s essential to avoid the food that causes the reaction. You should also consider seeing a healthcare professional.

Summary

Vitamin C may help treat seasonal or environmental allergies, which have symptoms including allergic rhinitis, sinus congestion, and asthma. However, there’s no evidence it can prevent these, or treat them as effectively as medication can.

The dose of vitamin C healthcare professionals most commonly use for allergic rhinitis is 2,000 mg per day (9).

The Recommended Dietary Allowance (RDA) for vitamin C is 90 mg per day for men and 75 mg per day for women (12).

Because this vitamin isn’t stored in your body, there’s minimal risk of toxicity. As such, it’s fairly safe to take higher doses in supplement form. Your body excretes any excess in your urine (12).

Note that for some people, vitamin C doses over 2,000 mg may cause nausea, vomiting, or diarrhea. Your gastrointestinal (GI) tract limits the amount of vitamin C it absorbs. For this reason, excess amounts can have a laxative effect (12).

Because of the risk of these side effects, the recommended upper limit is 2,000 mg per day (9, 12).

If you’re worried about side effects, start slowly and increase the dose over a few days to see how well you tolerate it.

You can also take it in smaller doses several times a day. This might also help you absorb more. At an oral dose of 1,000 mg, you only absorb about 50% of a vitamin C supplement (12).

Some functional and integrative healthcare professionals administer IV infusions of vitamin C.

This way of delivering vitamin C straight into the bloodstream bypasses your GI system. Healthcare professionals can give IV vitamin C in very high doses without the GI side effects.

Summary

The vitamin C dose healthcare professionals most commonly use for allergic rhinitis is 2,000 mg per day. Vitamin C has a very low risk of toxicity, but doses higher than this might cause GI side effects.

If you have any allergies that cause severe symptoms, it’s important to work with your healthcare professional and not rely solely on vitamin C to manage them.

You can certainly ask about using vitamin C as a complementary therapy, though.

Experts consider vitamin C supplements safe for most people to use. However, be aware they may interact with some medications.

Specifically, vitamin C might reduce the effectiveness of radiation, chemotherapy, and some cholesterol-lowering medications (12).

Vitamin C enhances the absorption of iron. That’s not a problem for most people.

However, it may be an issue if you have a condition called hemochromatosis, in which too much iron accumulates in your body. In this case, consuming too much vitamin C may cause you to accumulate more iron, which may damage tissues (12).

Finally, you should be cautious with vitamin C supplements if you have a kidney condition or are prone to kidney stones. A high intake of vitamin C could potentially contribute to forming kidney stones (12).

Before taking high doses of vitamin C, or any other nutritional supplement, it’s always a good idea to discuss the pros and cons with your healthcare professional.

Summary

Even though vitamin C is a safe supplement for most people, it might not be right for you if you take certain medications or have certain health conditions. It’s always wise to discuss supplements with a healthcare professional.

Vitamin C acts as a natural antihistamine by reducing the amount of histamine your body produces in response to an allergen.

It might help reduce mild symptoms like sneezing, runny nose, congestion, and watery eyes due to allergic rhinitis. It might also help protect your lung function and reduce the risk of asthma attacks.

There’s little risk of taking too much vitamin C, and most people can take up to 2,000 mg per day. However, there’s a chance of GI side effects, such as nausea or diarrhea, at oral doses higher than this.

Before taking vitamin C or any nutritional supplement, it’s always a good idea to talk with your healthcare professional to make sure there are no potential issues.