What Is Vitamin C?

Vitamin C, or ascorbic acid, is an important nutrient found in many foods, such as oranges, red and green peppers, and kiwi. It is also available in supplement form.

Vitamin C has many important functions in the body, including:

• Synthesizing collagen, an important protein
• Healing wounds
• Repairing and maintaining cartilage, bones, and teeth
• Functioning as an antioxidant, meaning it neutralizes free radicals—unstable molecules that can damage cells at the genetic level

Historically, vitamin C was used to prevent or treat scurvy, a condition caused by vitamin C deficiency. Today, vitamin C is considered an "immune booster" and is often taken as a cold remedy. There is some evidence that taking it may shorten the length and severity of a cold.

This article discusses the uses of vitamin C, possible side effects, and dosage.

What Is Vitamin C Used For?

Vitamin C has been used to treat and prevent many conditions, including:

• Asthma, a chronic lung disease
• Bronchitis, an inflammation of the air passages in the lungs
• Cancer
  
• Cataracts, a condition where the lens of the eye becomes cloudy
• Chronic or long-term pain
• The common cold
• COVID-19
• Gastritis, an inflammation of the stomach
• Glaucoma, a condition of the eyes
• Heart disease
• High blood pressure
• Osteoarthritis, a degenerative disease of the joints
• Parkinson's disease, a condition of the nervous system

There is not much evidence to support most of these uses. There have, however, been a few promising studies.

Vitamin C Deficiency

An estimated 8.4% of the U.S. population does not get enough vitamin C. In severe cases, this may lead to scurvy, although the condition is rare in the United States.

Symptoms of scurvy include:

• Bruising
• Bleeding gums
• Weakness
• Fatigue
• Rash

Vitamin C deficiency is treated with vitamin C supplements. Some symptoms improve within the first 24 hours of treatment. Others may take a few weeks to go away.

In severe cases, the bones may be affected. When this happens, surgery may be needed.

Common Cold

There is mixed evidence for the use of vitamin C to treat or prevent the common cold.

A 2016 review found little evidence that daily vitamin C supplements can prevent colds. One exception may be in people whose bodies are under constant physical stress, such as athletes.

Studies have, however, found that daily vitamin C supplements may help make colds less severe and decrease their length.

Amongst the research:

• The 2016 review noted that vitamin C taken within the first 24 hours of cold symptoms seemed to shorten how long people were sick.
• A 2013 study suggested that a daily dose of 1,000 to 2,000 milligrams (mg) of vitamin C can shorten a cold by 8% in adults and 14% in children.

Vision Loss

Age-related macular degeneration (AMD) is an eye disorder. People with this condition have vision loss.

A large 2001 study looked at vitamin C's role in the progression of AMD. Participants received daily vitamin C supplements along with vitamin E, beta-carotene, zinc, and copper.

The study concluded that the supplements slowed the progression of AMD by 25%. They also helped slow the loss of visual sharpness and clarity by 15%.

Unfortunately, researchers can't be sure that the vitamin C itself made a difference. Since this study, a number of other studies have not found that vitamin C alone is helpful for AMD patients.

In addition to this, vitamin C has been studied as a method for preventing certain types of cataracts. A 2014 review found that 135 mg of daily vitamin C may be useful in this regard. This study also found that doses of at least 363 mg could reduce the risk of developing cataracts by 57%. 

High Blood Pressure

Vitamin C has long been used as a treatment for high blood pressure, or hypertension. Unfortunately, research suggests it may not be as helpful as once thought.

The reason for vitamin C's effect on blood pressure isn't well understood. It is thought that high doses of vitamin C may have a mild diuretic effect. That means it may help reduce water in your body, which can lower blood pressure.

Heart Disease and Cancer

Research suggests that vitamin C may help prevent heart disease and cancer.

Antioxidants like vitamin C do seem to reduce the oxidative stress associated with these diseases. This can happen when there are too many free radicals in your body.

A 2013 review found that vitamin C may help prevent stroke. This was mostly true for people who had low intake of vitamin C before treatment or were at high risk for stroke.

The review only found this benefit for vitamin C from food sources, not from supplements.

A 2013 review noted that high intake of vitamin C does seem to be associated with a lower risk of certain cancers. It also noted that intravenous vitamin C has been shown to improve outcomes for cancer patients.

Many experimental cancer therapies include using vitamin C along with other treatments, however. This makes it hard to know how much of an impact vitamin C has on its own.

Possible Side Effects

Vitamin C is generally considered safe. High doses, though, can cause side effects. These may include:

• Heartburn
• Nausea
• Headaches
• Stomach cramps
• Diarrhea
• Kidney stones

Doses over 2,000 mg are considered extreme. These doses may increase the risk of severe diarrhea and kidney stones.

You can safely take lower doses of vitamin C during pregnancy. A dose of 85 to 120 mg per day is a common recommendation. Excessive use may cause harm to your unborn baby.

Vitamin C can also raise your blood sugar. Use vitamin C with caution if you have diabetes. In older women with diabetes, doses of more than 300 mg daily may increase the risk of death from heart disease.

Drug Interactions

If you take estrogen or estrogen-based contraceptives, vitamin C may increase the risk of hormonal side effects. This is because vitamin C may slow the rate at which estrogen leaves your body.

Vitamin C may also interact with the antipsychotic drug Prolixin (fluphenazine) and reduce the concentration of the drug in your bloodstream. This may make the drug less effective.

Vitamin C supplements can also make certain chemotherapy drugs less effective.

Let your doctor know if you are taking or planning to take vitamin C with any of these types of medications.

Dosage and Preparation

The recommended dietary allowance (RDA) of vitamin C is as follows:

*There are two important caveats to these recommendations:

• People who smoke should take an additional 35 mg per day.
• If you've been diagnosed with a vitamin C deficiency, you need to take between 100 to 200 milligrams per day until a blood test shows normal levels of vitamin C.

Vitamin supplements are available in the following forms:

• Tablets
• Capsules
• Chewable tablets
• Gummies
• Dissolving powders and tablets

You may also see different types of vitamin C, including:

• L-ascorbic acid, typically derived from corn
• Supplements that combine vitamin C with minerals such as sodium or calcium
• Citrus bioflavonoids
• Rose hips

No one form is more effective than another. There are also no differences between the effectiveness of the different types of vitamin C.

Are Vitamin C Supplements Necessary?

It is always best to get your nutrients from food rather than supplements.

Fruits and vegetables, especially citrus fruits, are good sources. Foods naturally rich in vitamin C include:

• Raw red peppers: 95 mg per 1/2-cup serving
• Orange juice: 90 mg per 3/4-cup serving
• Orange: 70 mg per one medium fruit
• Kiwi: 64 mg per one medium fruit
• Raw green peppers: 60 mg per 1/2-cup serving
• Cooked broccoli: 51 mg per 1/2-cup serving
• Strawberries: 49 mg per 1/2-cup serving
• Cooked Brussels sprouts: 48 mg per 1/2-cup serving
• Tomato juice: 33 mg per 1/2-cup serving
• Cantaloupe: 29 mg per 1/2-cup serving

If you don't get enough vitamin C from what you eat, a supplement can help get you to the right levels.

What to Look For

All supplements should be purchased with caution. You can make sure you're getting a safe, high-quality supplement by choosing products that have been tested and certified by a third party.

Look for products certified by an independent certifying body like U.S. Pharmacopeia, ConsumerLab, or NSF International.

An ordinary L-ascorbic acid supplement may be a better value than supplements that contain other ingredients.

Summary

Vitamin C has been used to treat many conditions. It is the best treatment for vitamin C deficiency. It may also help shorten the length and severity of the common cold. Other uses have less conclusive supporting evidence.

Food is the best source, but a supplement can help you meet your goals if needed. Speak to your doctor before taking one. The right dosage depends on several factors, and vitamin C can interact with certain drugs.