Possibly, but it probably doesn’t help very much, according to several scientific studies.
A 2004 review from Australian National University found that a "routine mega-dose [of vitamin C to prevent disease] is not rationally justified for community use," though the authors noted that vitamin C supplements could help people who have overexerted themselves physically and for individuals who spend significant time in cold environments. The authors revisited their research in 2013, but their conclusions didn't change.
Some studies have shown modest benefits for vitamin C supplementation. In 2013, a scientific review found that taking 200 milligrams of vitamin C every day during a cold could help reduce the duration of symptoms in adults by an average of 8 percent—but to get the best possible benefits, you’d have to consume vitamin C before you show symptoms.
What to Know About Vitamin C Supplements
Another important point: Most physicians recommend getting vitamin C through foods and liquids, not through dietary supplements. The FDA doesn't regulate supplements as strictly as it does medicine, and supplements' labels sometimes misstate their ingredients; vitamin-rich foods deliver other crucial nutrients, and they're typically much cheaper than supplements.
Mega-doses of vitamin C aren’t helpful, since our bodies aren’t equipped to store massive amounts of the vitamin. Excessive vitamin C passes out of the body in the urine, so most adults won't see any benefit from 400 milligrams or more of vitamin C. For perspective, an orange contains about 69.7 milligrams of vitamin C. By adding more fruits and vegetables to your diet, you can easily meet your daily threshold for vitamin C (65 to 90 milligrams for adults) without bothering with supplements.
Staying Healthy During Cold and Flu Season
If you're looking for a supplement that could boost your immune system during flu season, consider zinc. A 2011 article in The Journal of Family Practice found "convincing evidence from 13 randomized placebo-controlled trials that taking zinc soon after the onset of symptoms of the common cold significantly reduces both the duration and severity of symptoms." Unfortunately, zinc also tends to have some side effects; over-the-counter supplements can cause nausea, and long-term use can interfere with copper metabolism.
With that said, here's a simple game plan for flu season: Eat a balanced diet, wash your hands regularly, and consider taking a zinc supplement if you come down with an illness. Ask your physician before making any major changes to your diet or lifestyle—and if you're really worried about illness, consider getting a flu shot.