By Roberta H. Anding, MS, Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Does taking large quantities of vitamin C boost your health and protect you from illnesses, most notably the common cold? Professor Anding weighs in.

Effects of Excess Vitamin C

Although some people believe they should take up to 2,000 milligrams of vitamin C per day for optimal health, taking excess vitamin C is not only unnecessary but can even be harmful. While vitamin C is relatively nontoxic as it’s water-soluble, it can still lead to side effects. 

To put things in perspective, the recommended daily allowance (RDA) for males in the age range of ages 19 to 90 is around 90 milligrams and about 75 milligrams for females in the same age group. We now have a new concept in clinical nutrition of an upper limit of intake. For vitamin C, the upper limit should be about 2,000 milligrams. Even lower amounts can be harmful for some people, though.

Some studies suggest a possible link between excess vitamin C and an increased prevalence of kidney stones. Vitamin C is metabolized to a urinary product called “oxalate,” and many of the kidney stones that people develop are oxalate-based stones. 

As with most things in clinical nutrition, there’s some conflicting research, but in general, keep in mind these individuals are consuming 1,000 milligrams of vitamin C or more a day. Additionally, some African-Americans as well as those of Jewish heritage and Asian descent may actually have a genetic metabolic deficiency that, with excessive vitamin C intake, creates hemolytic anemia, where the red cells rupture and lose their iron content.

In individuals with iron deficiency, megadosing vitamin C or taking up to 10 times the amount that you need can destroy significant amounts of vitamin B₁₂. Too much vitamin C actually affects the bioavailability of vitamin B₁₂. In healthy people, excess vitamin C can cause bowel irritation and chronic diarrhea.

Blood Tests and Vitamin C

“To me, as a clinical dietician, one of the most disconcerting things with excess vitamin C is it can interfere with other diagnostic tests,” Professor Anding said. 

If you’ve ever had a test called “fecal occult blood in the stool” where the doctor is trying to decide whether you’re going to be a candidate for a colonoscopy, he may check if you’re losing any blood in your stool. If you’re taking an excess of vitamin C, it can interfere or react with the reagent that’s testing for blood, showing up as blood in your stool and making you eligible for a colonoscopy you don’t actually need.

Vitamin C can also interfere with the test for glucose or sugar in your urine. The test will show that you have sugar in your urine when in fact it’s just the excess amount of vitamin C. 

Does Vitamin C Prevent Colds?

Many people who take excess vitamin C believe that it will protect them against illnesses, particularly colds. Scientists investigated whether vitamin C could shorten the length of the common cold in 30 studies involving almost 10,000 respiratory episodes—individuals who have been sick—contributing to a pooled research study.

Individuals taking excess vitamin C to prevent upper respiratory infections can reduce the duration of a cold by about 8% for adults and 13% to 14% for children. However, it doesn’t necessarily prevent the cold or lessen the intensity. This goes against Nobel Laureate Dr. Linus Pauling’s belief that vitamin C would prevent the common cold. 

One area where vitamin C can be beneficial is age-related eye disease (AREDS). The AREDS study indicated that taking 500 milligrams of vitamin C in addition to other dietary compounds that function as antioxidants and zinc can lead to the reduction of that disease. You can now find supplements based on that AREDS study.

Overall, then, there are some cases where taking excess vitamin C can be beneficial to your health. In most cases, though, it is not necessary and can even lead to harmful side effects. As always, consult with your doctor first.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.