Our survey asked specifically about 13 types of dietary supplements and allowed respondents to list any others they used as flu remedies. Just 9 percent of those surveyed said they planned to try elderberry or n-acetyl cysteine this winter, two supplements that, as of November 2010, were rated "possibly effective" against flu symptoms. Our analysis of supplements was based on information from the Natural Medicines Comprehensive Database (NMCD), a compendium from an independent research group that evaluates the evidence for safety and effectiveness of nutritional supplements. Both elderberry and n-acetyl cysteine are available as single ingredient supplements or as part of multi-ingredient formulations.
Clinical research suggests that some elderberry extracts might help relieve flulike symptoms within 2 to 4 days of treatment for most patients. Supplements of n-acetyl cysteine, an amino acid derivative that's also available as a prescription drug for lung problems due to pneumonia and other causes, might also help reduce flu symptoms. An Italian clinical trial published in 1997 of 262 mostly elderly people found that those who received the supplement during winter (the height of flu season) had significantly fewer flu infections and milder symptoms than those given the placebo.
It's unclear whether the people surveyed chose those supplements because they knew there was some evidence of efficacy. What raises doubts is that elderberry and n-acetyl cysteine users said they planned to take a total of eight types of supplements as flu remedies, about twice the average number most supplement users planned to take this flu season, our survey found. Cost isn't the only downside to a "might as well take them all" approach to supplement taking: Supplements have side effects and may interact with prescription and over-the-counter medications.
NMCD rates n-acetyl cysteine supplements as "likely safe," and elderberry extracts as "possibly safe" for short-term use, but it's important to first check with your doctor or pharmacist. Most supplements haven't been studied in pregnant or nursing women, who should avoid them. Elderberry plants and fruit contain a cyanide-producing chemical. Consuming the plant's leaves, flowers, or unripened or uncooked fruit can result in nausea, vomiting, and diarrhea. Ingestion of large quantities may cause serious toxicity. People with allergies to grass pollen should also avoid elderberry extracts because of the potential for an allergic reaction.
Other supplements used as flu remedies, such as vitamin C and zinc, have been found to be possibly effective against the common cold, according to the NMCD, but not against the flu. While preliminary evidence published in 1997 suggested that zinc with selenium might fight flu infections in older, institutionalized patients, the evidence was weak, and the NMCD has rated zinc supplements as "possibly ineffective" against influenza.
The survey did not ask specifically about vitamin D supplements, but in a randomized clinical trial involving 334 Japanese school children, published in the May 2010 American Journal of Clinical Nutrition, children who took vitamin D supplements had a significantly decreased chance of contracting influenza A. Only 18 of the 167 children taking vitamin D3 supplements got influenza A, compared with 31 of 167 children in the placebo group. There was, however, no significant difference between the two groups in catching influenza B. Children who had not previously taken vitamin D supplements showed the most benefit in this trial, suggesting that they might have been vitamin D deficient.
The NMCD concluded that vitamin D supplements are "possibly effective" in reducing the chance of catching the flu, and "likely safe" when used appropriately.
Children and adults with severe flu symptoms—including severe diarrhea, vomiting, difficulty breathing, and high fevers—should seek medical attention and not just rely on supplements.