(CNN) Can vitamin C and zinc help you fight off Covid-19?
No, not even at high doses, according to the first randomized clinical trial to test the two supplements under medical supervision.
Despite the popular use of vitamin C and zinc to fight off or lessen the severity of viral colds and flu, the new study, published Friday in JAMA Network Open, found the two supplements were of no benefit to people isolating at home with Covid-19.
In fact, the findings were so unimpressive that the study was stopped early.
"Unfortunately, these 2 supplements failed to live up to their hype," wrote Dr. Erin Michos of John Hopkins and Houston Methodist's Dr. Miguel Cainzos-Achirica, in an accompanying editorial.
The clinical trial gave high doses of each supplement alone and in combination to one of three groups of 214 adults who were recovering at home. A fourth group got standard care, such as rest, hydration and fever-reducing medications, but no supplements.
"High-dose zinc gluconate (zinc), ascorbic acid (vitamin C), or both supplements did not reduce SARS-CoV-2 symptoms," according to Cleveland Clinic cardiologist Dr. Milind Desai and a team from Cleveland Clinic.
The high doses, however, did cause some unpleasant side effects for patients taking the supplements.
"More adverse effects (nausea, diarrhea, and stomach cramps) were reported in the supplement groups than in the usual care group," wrote Michos, an associate professor of medicine at Johns Hopkins University School of Medicine, and Cainzos-Achirica, an assistant professor of preventive cardiology at Houston Methodist.
Many Americans turn to vitamin C and zinc supplements to fight off viral colds and flu.
Vitamin C is a recognized antioxidant, and plays an essential role in supporting the immune system. Even though it has not been shown to prevent illness, other research has found vitamin C can shorten colds by 8% in adults and 14% in kids.
Using vitamin C after cold symptoms start, however, doesn't appear to be helpful, according to the National Institutes of Health.
Zinc may help a cell's ability to fight infection, the study said, "while there is evidence that zinc deficiency increases pro-inflammatory cytokines and decreases the production of antibodies."
But what does that mean in real life? If taken within 24 hours of the very first signs of a cold, zinc may reduce the length of a cold by only one day, a a review of 13 studies found.
There is a down side, too. Taking over 2,000 milligrams of vitamin C a day can cause heartburn, stomach cramps, nausea, vomiting, diarrhea and headaches. The average daily recommended amount of vitamin C is 75 milligrams for adult women and 90 milligrams for men.
Over 40 milligrams of zinc each day can cause dry mouth, nausea, loss of appetite and diarrhea, plus it can have a nasty metallic taste.
Long-term users can have "low copper levels, lower immunity, and low levels of HDL cholesterol (the 'good' cholesterol)," according to the NIH. In 2009, the FDA warned the public against using zinc nasal sprays because they were linked to more than 100 cases of loss of smell.
Scientists are continuing to explore the use of vitamins and supplements in the treatment of Covid-19.
Randomized trials are currently underway to see if vitamin D supplements can help -- in addition to supporting healthy bone growth, vitamin D has anti-inflammatory properties.
Researchers in the US and China are using intravenous doses of vitamin C to determine if it might help people with respiratory failure severe enough to be put on a ventilator.
And more studies are in progress to see if Covid-19 can be prevented by supplements such as vitamins C and D and zinc.
Based on this study, however, the use of vitamin C and zinc to shorten the duration of Covid-19 appears to be useless.