Everyone knows that bed rest, plenty of fluids and Grandma's chicken soup can soothe (if not cure or prevent) a winter cold.

But what about supplements that claim to prevent colds or reduce the severity of their symptoms?

"While a few supplements may work, for many the evidence is mixed or very preliminary," says Tod Cooperman, CEO of ConsumerLab.com, an independent lab that recently examined research on natural or herbal supplements that claim to treat the common cold.

One of the most tested and promising is zinc, a mineral shown to reduce the severity and frequency of colds if taken by lozenge (not pills) and in the correct dosage of about 13 to 20 mg every three to four hours.

However, Cooperman warns that zinc should not be ingested longer than a week or in excessive doses, since it can cause an upset stomach.

"Taking too much zinc can also reduce the absorption of copper," says Cooperman, "which can cause more colds."

Cooperman recommends Cold-EEZE, which includes "proper instructions on how to take it." He advises avoiding all zinc nasal sprays, which have been shown to affect the sense of smell.

Certain combinations of probiotics also exhibited promise in treating and preventing colds. In one study, probiotics taken over three months reduced the risk of catching a cold by 12 percent (55-67%) in healthy adults when compared with a placebo.  Probiotics also cut symptoms, such as acute upper respiratory tract infections.

Unlike taking megadoses of vitamins or minerals, probiotics are generally safe. One brand that has been tested for colds is Metagenics UltraFlora with lactobacilli.

Several other supplements disclosed mixed results.

One large study found that taking a branded liquid Echinacea three times daily over the cold season "modestly reduced the risk of getting a cold." When taken five times daily during a cold, it cut the amount of pain medication, such as aspirin, that needed to be used during a cold.

"Echinacea sometimes works and sometimes doesn't," says Cooperman.

When it comes to Vitamin C, "if you aren't deficient in Vitamin C, there isn't much evidence that it can prevent a cold," he says. While taking modest doses of Vitamin C – roughly 100 mg daily -- during the cold season may reduce the severity of a cold, Cooperman cautions against taking megadoses of 1000mg or above, which have shown no proven benefit.

In several clinical studies, the herb andrographis staved off colds and reduced symptoms such as earache, sleeplessness, nasal drainage and sore throat. However, ConsumerLab.com notes that much of the research was done on a branded formula – Kan Jang from Swedish Herbal Institute -- that contains andropgraphis and Siberian ginseng. While generally considered safe and well tolerated, the herb may not be suitable for people with autoimmune diseases or gallbladder disease.

And speaking of ginseng, very preliminary studies of a branded American ginseng extract suggest that it might help prevent colds.

While taking supplements may offer limited relief, in the end Cooperman advises that "eating nutritionally square and getting enough sleep" may be the best treatment.