There is evidence that certain supplements can reduce the chance of getting a cold and/or reduce the severity of a cold.
Zinc can directly kill cold viruses and, when taken as a lozenge (as opposed to a pill), it may significantly reduce the duration and severity of cold symptoms. However, it needs to be taken properly and at the correct dosage and it should not be taken for more than about a week due to concerns about excessive zinc.
The effects of probiotics for cold prevention in adults is mixed. A certain combination of probiotics taken for 3 months was found to reduce the risk of catching a cold by about 12% compared to placebo in one study, although another study showed no benefit with the same combination for cold prevention. In addition, a specific single strain of probiotic had no benefit for cold prevention in adults when taken for 4 weeks before exposure to a cold virus. Research on probiotics for cold prevention among children seems more promising, but it is still unclear which, if any, strain of probiotic is most beneficial and in what amount.
Unless you are deficient in vitamin C, there's not much evidence that it can help prevent a cold. However, if taken routinely during cold season, it may reduce the severity of a cold.
Evidence for echinacea has been mixed. In general, this herb may be more effective for reducing the risk of catching a cold, rather than reducing symptoms of an existing cold. One large study found that taking a branded liquid echinacea three times daily during cold season modestly reduced the risk of getting a cold; if taken 5 times daily during a cold, it reduced the amount of pain medication, such as aspirin, used during colds.
Andrographis (Andrographis paniculata), sometimes referred to as "Indian echinacea," is an herb which has been shown in several clinical studies to help prevent colds and reduce cold symptoms such as earache, sleeplessness, nasal drainage, and sore throat. Much of the research on this herb has been conducted on a branded formula (Kan Jang from Swedish Herbal Institute) which contains a combination of andrographis and Siberian ginseng. Although generally considered to be safe and well-tolerated, elevated liver enzymes and rarely, anaphylactic reactions have been reported. Andrographis should be used with caution in people taking immunosuppressive drugs, and people taking blood-thinning or blood-sugar lowering medication.
Vitamin D supplements may help reduce the occurrence of colds and upper respiratory infections, but only in people who do not already have sufficient blood levels of vitamin D (20 ng/mL). In these people, moderate daily doses may be more effective than extremely large, periodic doses.
For pregnant and nursing women, getting 400 mg of DHA from fish or fish oil supplements has been shown to reduce colds in their infants, but only for infants of women with otherwise low intake of DHA.
Preliminary studies of American ginseng extracts suggest that it might help prevent colds, but there isn't enough good evidence to support this use. Other studies have shown that Asian ginseng may boost the immune system, but the evidence is mixed on whether or not this leads to fewer colds.
Preliminary evidence suggest that beta glucan (a type of soluble fiber from sources such as yeast or grains) may improve certain measures of immune system function — although the evidence is mixed as to whether or not this translates into a reduced risk of a cold. For example, a study in otherwise healthy adults who had recurring colds found that 900 mg of beta glucan (Yestimun, Leiber GmbH, Germany) taken daily for 4 months reduced the number of colds by 25% compared to placebo (Auinger, Eur J Nutr 2013), and a daily dose of either 250 mg or 500 mg of another beta glucan product (Wellmune WGP) was found to reduce the incidence of respiratory infections in runners when taken for one month following a marathon (Talbott, J Sports Sci and Med 2009). However, another study found 500 mg of Wellmune WGP did not decrease the incidence of respiratory infections in adults compared to placebo (Feldman, J Appl Res 2009).
There is some evidence that a branded fermented yeast product containing beta glucan (EpiCor, Embria Health Sciences) may modestly help to increase certain measures of immune function and reduce the occurrence of colds.
NAC (N-acetyl-cysteine) has been shown to thin mucus but there is little evidence that oral supplements help with colds. Oral NAC, however, may decrease the frequency of chronic bronchitis.
There is some evidence that regular use of garlic may help prevent colds. One study found that taking a garlic supplement daily throughout the winter reduced the likelihood of catching a cold by almost two-thirds, and those who caught a cold recovered about one day faster than those who did not take garlic (Josling, Adv Ther 2001).
Glutathione is an antioxidant produced in the liver which is important for immune function. It is sometimes promoted to boost the immune system or help to ward off colds. To date, one small study found that a dose of 250 mg or 1,000 mg per day for six months reduced oxidative stress compared to placebo - although there does not appear to be any evidence that it prevents colds or reduces cold symptoms.
Some people have reported taking melatonin to help boost the immune system and promote sleep while having a cold. Very preliminary evidence suggests melatonin has some antiviral properties (Vielma, Acta Trop 2014), although there does not appear to be any research specifically on colds in people.
Bromelain has been reported to reduce nasal inflammation and improve breathing difficulty in people with sinusitis. (ConsumerLab.com's Review of Digestive Enzyme Supplements includes products containing bromelain).
Several clinical studies suggest elderberry extract may help reduce and shorten the duration of symptoms of colds and flu. However, most of these studies are small and company-funded, so more research is needed. Much of the research to-date has used a particular elderberry extract, Sambucol. None have shown elderberry to prevent colds or flu. Although rare, be aware that a case of acute pancreatitis was reported in 2019 in a 51-year-old man after he took two to three doses of elderberry extract (Sambucol). He reported having similar symptoms (abdominal pain, nausea, and vomiting) years prior after taking one dose of the same extract. His condition improved after treatment with steroids (Weissman, Dig Dis Sci 2019).
Astragalus is often promoted to help fight colds when taken at the onset of a cold. Preliminary evidence suggests it may help to stimulate the immune system, although there do not appear to be studies in people on the effects of astragalus on cold symptoms.
Olive leaf extract is promoted to boost the immune system and fight colds. One study in high school athletes found that taking olive leaf extract did not prevent colds, but it reduced the duration of illness that occurred by about 28% (Somerville, Nutrients 2019).
Although some people have reported preventing colds with Oscillococcinum, a homeopathic treatment, there is little evidence for this use. There are many other homeopathic remedies proposed for preventing or treating colds, but little quality evidence to support these remedies.
There is no evidence that honey can prevent or shorten colds, but it may relieve symptoms better than some over-the-counter medicines. An analysis of data from 14 clinical trials found that honey improved overall symptoms and reduced the frequency and severity of cough compared to usual care. In particular, honey reduced cough frequency better than the antihistamine diphenhydramine (ex: Benadryl) and comparable to the cough suppressant dextromethorphan (Ex: Robitussin). Most of the reviewed studies gave honey in doses of 0.5 to 2 teaspoons per day to children and about 1 tablespoon per day to adults, often at bedtime, but it remains uncertain if these doses are optimal. Most of the studies included in the analysis used pure honey, but some combined honey with other ingredients such as milk or coffee (Abuelgasim, BMJ Evid Based Med 2020). While there is interest in manuka honey for health purposes, none of the studies included in the analysis used manuka honey. (Note: Be aware that infants younger than 12 months should not consume honey of any kind due to the risk of infant botulism. Topical administration is also inadvisable for infants.)