COUNTLESS immune boosters fly off the shelves year-round, particularly in cold and flu season. But even some of the most popular immune supplements haven't been tested against cold and flu in well-designed clinical trials.
No clinical trials, for example, have been conducted on goldenseal, a purported immune-booster sometimes used to fend off the common cold -- and there's evidence the herb may interfere with blood pressure drugs, decrease the activity of anticoagulants and hamper absorption of vitamin B. Goji, sometimes known as lycium, is a popular immune booster based on evidence from lab and animal studies, and some human research in China -- but no clinical trials have examined its ability to keep flu or colds at bay. The same is true for supplements derived from the tropical fruits noni and mangosteen.
A few herbs popular for other indications have recently shown preliminary promise for colds and flu. Green tea has been the focus of countless cancer and heart disease studies in recent years; just last fall, colds and flu were added to the list. A November 2007 article in the Journal of the American College of Nutrition reported that adults who took a green tea capsule twice a day for three months had 23% fewer colds and cases of the flu than people who took a placebo.
Andrographis paniculata, a fever remedy in traditional Chinese medicine, has been shown to kill microorganisms in lab studies. Combined with Siberian ginseng in a formula called Kan Jang, the herb reduced cold and flu symptoms in clinical trials conducted in Russia and Europe. In several large studies, it shortened flu recovery time slightly and reduced nasal secretion, congestion and coughs better than placebo. The downside: Kan Jang can cause headaches and fatigue in people not already suffering those symptoms because of the flu, and some people may be allergic to it.