Vitamin C may modestly ease cold symptoms when taken regularly, but evidence does not support it as a general way to prevent colds.
As winter arrives and cold season begins, vitamin C products often move to the front of pharmacy and supermarket displays, reinforcing a familiar message: taking extra vitamin C might help keep a cold away.
But the belief has a surprisingly specific history.
Much of its popularity can be traced to a two-time Nobel Prize winner, a bestselling book, and scientific claims that later came under substantial criticism.
A Nobel winner helped fuel the myth
American chemist Linus Pauling received two Nobel Prizes. He won the 1954 Nobel Prize in Chemistry for his research on chemical bonds, followed by the 1962 Nobel Peace Prize for campaigning against nuclear weapons testing.
Pauling was neither a dietitian nor an infectious disease physician. Even so, in 1970 he published a book about vitamin C and the common cold, which is widely credited with driving a dramatic increase in vitamin C sales that continues to influence consumer habits today.
In the book, Pauling recommended taking roughly 1,000–2,000 milligrams of vitamin C each day for good health and even larger amounts to prevent colds.
His recommendations were said to have initially drawn on a placebo-controlled study involving children at a ski camp in the Swiss Alps. He later examined four additional studies conducted after making those claims.
However, his interpretation of the evidence has since been widely criticized. Critics have pointed to mathematical errors, reliance on four low-quality trials, and excessive emphasis on the study involving children.
Regular vitamin C offers limited benefits
The short answer is not really. Multiple studies conducted since the ones Pauling analyzed show that if there is an effect of vitamin C on colds, it’s minimal. Even then, any marginal benefits are only under certain circumstances, such as if you’re an athlete or in the army.
One study, published in 2023, was a meta-analysis that combined the results of ten placebo-controlled trials. These looked at the effect of vitamin C on the severity and duration of colds. In total, these trials involved 2,736 healthy people (adults, children, and athletes) who regularly took at least 1,000 mg of vitamin C a day, not just when they had a cold.
Overall, the researchers found people who took vitamin C had a 15% reduction in severe cold symptoms compared to those who didn’t take it. This was expressed as the number of days “confined indoors,” which the authors acknowledged as a limitation. Vitamin C did not affect how long mild symptoms lasted.
A meta-analysis in 2013 found taking lower doses (at around 200 mg a day) did not decrease the number of colds in the general population. However, there were fewer colds for athletes and army personnel.
Similar to the other study, regular vitamin C supplements showed a reduction in severity of symptoms in children and adults by 8–14%. However, taking vitamin C at the start of the cold had no benefit.
So, from these two meta-analyses, Pauling’s original advice is overstated. There is no evidence vitamin C will reduce your risk of catching a cold.
There is some evidence it will reduce how long severe symptoms last, but on average only by about 10%. So if severe symptoms from a cold last for five days, it will reduce severe symptoms by about 12 hours. If the severe symptoms last for 24 hours, they will reduce by about two-and-a-half hours.
Taking vitamin C when symptoms start will have no effect. You need to be taking it regularly, even before you get sick.
High doses far exceed daily needs
Pauling’s recommendation for good health, at around 1,000–2,000 mg a day, is around the upper limit of what’s now considered safe to consume.
Australia has not set an upper limit for vitamin C, due to inconclusive evidence. But guidelines refer to other expert recommendations for adults not to consume more than 1,000 mg a day. In the United States, the upper limit for adults is 2,000 mg a day.
Pauling’s recommendation is also much higher than the recommended daily intake in Australia and other countries. For adults over 19 in Australia, this is 45mg.
More vitamin C is not always better
This is what you would find in half an orange, three to four florets of cooked broccoli or one-third of a glass of orange juice.
Taking high doses of vitamin C (over 1,000 mg a day) continually can increase your risk of adverse events such as:
- gastrointestinal problems, including diarrhea, nausea, stomach cramps, and bloating
- increased oxalate excretion, which may raise the risk of kidney stones
- increased iron absorption, which may cause tissue damage in people with undiagnosed hemochromatosis (an inherited disorder where the body absorbs too much iron from food)
- interactions with radiation therapy, chemotherapy, and cholesterol-lowering statin medications.
If you feel you would benefit from vitamin C supplements, discuss this with your GP. You should limit the dose to below 1,000 mg a day and be mindful of the side effects.
Remember, less than 4% of Australians are deficient in vitamin C.
Adapted from an article originally published in The Conversation.
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