BUST, say two recent studies from the National Center for Complementary and Alternative Medicine (NCCAM). The studies tested three different kinds in various doses and found they didn't prevent colds or flu, shorten their duration or make them less severe.
BUST. However, intranasal zinc (like Zicam) may help shorten a cold's duration, but experts say the jury's still out.
BOOST, in the sense that it may have a mild effect in drying secretions once you have a cold. Although the research and experts go back and forth on whether or not these substances work, studies on cold treatments have proven one thing: "If you're doing something you think works, it is likely to have a positive effect [placebo effect]," says Dr. Khazeni. Buyer beware, however: "The FDA doesn't regulate herbal or vitamin supplements, so you don't know if you're getting what it says on the package," says Dr. Khazeni. Be sure to talk to your doctor before using anything.