Garlic may also lower blood glucose levels and increase the release of insulin, so it should be used with caution by people taking drugs that lower blood sugar. People with allergies to plants in the lily family (including onion, leeks and chives) should avoid garlic. Pregnant women should avoid garlic in supplement form because it may increase the risk of bleeding.
In 1968, Linus Pauling, PhD, proposed the theory that people had individual requirements for various vitamins and some needed amounts higher than the recommended dietary allowances (RDAs). Pauling proposed that 1,000 mg of vitamin C daily could reduce the incidence of colds for most people. Since then, vitamin C has become a popular cold remedy.
A review by the Cochrane Collaboration examined whether vitamin C taken orally in doses of 200 mg or more a day could reduce the incidence, duration or severity of the common cold. The researchers analyzed 30 previously published studies (involving a total of 11,350 participants) that met their quality criteria. They found that vitamin C didn’t appear to prevent the common cold. There was a slight reduction in the length and severity of cold symptoms. It appeared to markedly reduce the risk of catching a cold in people involved brief, intense physical activity (such as marathon running or skiing), or in those exposed to cold temperatures.
Vitamin C in amounts over 2,000 mg may cause diarrhea, loose stools and gas.
Honey is a popular home remedy for cough and colds in many cultures. A new study in the Archives of Pediatrics and Adolescent Medicine provides the first evidence showing that honey may help to calm children's coughs and help them sleep better. Researchers gave 105 children with colds eith honey, honey-flavored cough medicine or no treatment. All of the children got better, but honey consistently scored best in parents' rating of their children's cough symptoms.
The researchers say that honey may work by coating and soothing an irritated throat and it’s believed to have antioxidant and antibacterial effects. Dark-colored honey, such as the buckwheat honey used in the study, is particularly high in antioxidants.
Honey isn't recommended for infants younger than one year old, because of the risk of botulism. Regular use of honey at night may also promote the development of cavities.
Although recent findings question the use of echinacea for colds and flu, it’s still one of the most popular herbs used today. A 2005 study by the National Center for Complementary and Alternative Medicine found that echinacea did little to prevent or shorten the common cold. There were many critics of the study, who say that the study shouldn't be used as evidence that echinacea doesn't work. The Cochrane Collaboration conducted a review of 15 studies on echinacea, however, and found that it wasn't more effective than a placebo at preventing colds.
Although there are several types of echinacea, the above-ground parts (the leaves, flowers and stems) of echinacea purpurea have subject to the most research.
Herbalists often recommend taking echinacea every two to three hours with a total daily dose of three or more grams per day at the first sign of symptoms. After several days, the dose is usually reduced and continued for the following week. Echinacea is also an ingredient in Airborne, a supplement containing vitamins and herbs that’s sold over the counter.
Although there are many types of ginseng, one cultivated in North America called Panax quinquefolius or “North American ginseng" has become popular as a remedy for colds and flu. Compounds called polysaccharides and ginsenosides are thought to be the active constituents in ginseng. One of the more popular ginseng products is Cold-fX.
Two studies tested Cold-fX in 198 nursing home residents, who received either Cold-fX or a placebo. There was no statistically significant difference in the number of people who contracted the flu and no difference in the severity or duration of the flu. The researchers analyzed the results of the two studies together and only then did the results show that Cold-fX reduced the incidence of the flu. Although it's popular and some people swear by it, large, well-designed, independent trials are needed to determine the safety and effectiveness of this product.
There is some concern that ginseng may reduce the effectiveness of "blood-thinning" (anticlotting or antiplatelet) drugs such as warfarin (Coumadin) or aspirin. It may interact with diabetes medications, antidepressants known as MAO inhibitors, antipsychotic drugs (e.g., chlorpromazine (Thorazine), fluphenazine (Prolixin), olanzapine (Zyprexa)), drugs that stimulate the central nervous system (used to treat conditions such as attention deficit hyperactivity disorder, narcolepsy, obesity, and heart conditions) and estrogen replacement therapy or oral contraceptives.
Ginseng root is thought to have estrogen-like properties and is usually not recommended for people with hormone-related conditions such as uterine fibroids, endometriosis and cancers of the breast, ovaries, uterus or prostate. People with heart conditions, schizophrenia or diabetes also shouldn’t take ginseng root unless under a doctor’s supervision. The manufacturer of Cold-fX indicates on their website that because their product isn't a whole plant extract but contains a certain compound found in ginseng, it doesn't have the side effects and safety concerns commonly associated with ginseng; although that's possible, there isn't published safety data confirming these claims.
