Answer: 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.
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, it should be noted this herb may not be suitable for people with certain autoimmune diseases or gallbladder disease.
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.
Overall, the evidence is mixed on whether garlic supplements are effective against colds. However, as discussed in our Encyclopedia article on garlic, one study found 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.
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.
There is some evidence that elderberry extract has antibacterial and antiviral activity, but a study of overseas travelers took either elderberry extract (600 mg per day for 8 days before travel and then 900 mg per day during travel for 4-5 days afterward) were just as likely to get a cold as those who took a placebo. However, among those who did get a cold, colds were 2 days shorter and symptoms were less severe than for those who took the placebo (Tiralongo, Nutrients 2016). The study was funded by the manufacturer of the extract (BerryPharma, Iprona AG, Italy). 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. However, despite laboratory studies suggesting it has some antiviral activity, there are no studies on the effects of olive leaf extract supplementation on cold symptoms in people.