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Cold and Flu Season Supplements: What the Evidence Says (Vitamin D, Zinc, Elderberry and More)

Cold and flu supplements do less than labels suggest. Zinc acetate or gluconate lozenges started within a day of the first symptoms shortened colds by about two days (grade B). Vitamin D did not clearly prevent respiratory infections in 40 trials (grade A), and elderberry rests on small, mixed trials (grade C). A yearly flu vaccine has stronger evidence than any of these; handwashing (grade B) and enough sleep (grade C) may also help. Never put zinc in your nose.

Published Updated How we grade evidenceEditorial policy

The short version

  • Zinc lozenges started early: colds about 2 days shorter, with more minor side effects. Grade B (Nault 2024; Hemilä 2017).
  • Vitamin D to avoid colds and flu: no clear protection overall across 40 trials. Grade A (Jolliffe 2025).
  • Vitamin C every day: no fewer colds for most people, but colds about 8% shorter in adults. Grade A (Hemilä 2013).
  • Elderberry: a few small trials, with mixed results. Grade C (Wieland 2021; Macknin 2020).
  • Probiotics: fewer people caught an upper airway infection. Grade B (Zhao 2022).
  • The flu vaccine: flu in healthy adults fell from 2.3% to 0.9% of people in a season. Grade A (Demicheli 2018).

A cold is mostly a runny nose and sore throat; flu comes on fast, with fever, aches and exhaustion, and more often leads to pneumonia. Many trials below counted "acute respiratory infections" (any new infection of the nose, throat or chest), so we say what each one measured.

Do zinc lozenges help a cold?

Grade B. Yes, if you start them early. A 2024 Cochrane review pooled 8 treatment trials (972 people): colds ended about 2 days sooner on zinc than on placebo (mean difference 2.37 days), though the reviewers rated the evidence low because the trials varied so much (Nault 2024). Most treatment trials used lozenges, which dissolve slowly in the mouth: in 7 lozenge trials giving more than 75 mg of zinc a day, colds were 33% shorter, with acetate and gluconate doing about equally well (Hemilä 2017). In the raw data of 3 zinc acetate trials (199 people), colds were 2.73 days shorter than the usual 7 (Hemilä 2016).

The researchers behind the acetate trials advise starting within 24 hours of the first symptoms and stopping within 2 weeks (Hemilä 2015); more than 100 mg a day did not work better (Hemilä 2017). Not every trial agrees: in a 2020 Finnish trial of 87 adults with colds, those given zinc acetate lozenges for 5 days recovered no faster than those on placebo (Hemilä 2020).

Grade B for daily zinc to avoid colds: little or no effect (risk ratio 0.93, 9 trials) (Nault 2024).

Never use zinc in your nose. Zinc nasal gels have taken away people's sense of smell: one clinic described 17 patients who lost it, often within 48 hours of sniffing the gel, and a review of 25 cases concluded the gel caused it (Alexander 2006; Davidson 2010).

Does vitamin D keep colds and flu away?

Grade A for no clear protection overall. The largest analysis, updated in 2025, pooled 40 trials against placebo, covering 61,589 people. People on vitamin D were about as likely to catch at least one respiratory infection as people on placebo (odds ratio 0.94, a range that includes no effect). Age, starting vitamin D level, dose size and how often it was taken made no clear difference (Jolliffe 2025). Small trials reported bigger benefits than large ones, a sign of publication bias (Autier 2025).

Grade C for people who are very low in vitamin D. A 2017 analysis of raw data from 25 trials saw the most benefit in daily or weekly takers who started under 25 nmol/L (odds ratio 0.30) (Martineau 2017), but later updates did not confirm that the starting level mattered (Jolliffe 2021; Jolliffe 2025). And in a UK trial of 6,200 people aged 16 and over, offering half of them a blood test plus 6 months of vitamin D if they were low did not cut respiratory infections (Jolliffe 2022).

If a test shows you're low, correct it for its own sake, but don't count on it to spare you a cold. Our database page says vitamin D "improves" cold symptoms; the large pooled analyses don't support that.

Does elderberry help a cold or the flu?

Grade C. The evidence is thin and mixed. A 2021 systematic review found only 5 randomized trials: elderberry may not reduce the risk of getting a cold, and may shorten colds and flu, but the evidence is uncertain (Wieland 2021). In 60 adults in Norway with flu-like illness, elderberry syrup for 5 days relieved symptoms about 4 days sooner than placebo (Zakay-Rones 2004). In 312 air travellers, colds were shorter but not fewer (12 against 17) (Tiralongo 2016). The most careful trial, in 87 emergency-room patients with lab-confirmed flu, found no benefit: symptoms eased after 5.3 days on elderberry and 4.9 on placebo (Macknin 2020). Our elderberry page now gives the same grade C.

Does vitamin C prevent colds or shorten them?

Grade A. It depends on how you take it. In the Cochrane review's community trials (10,708 people), taking vitamin C every day did not make people less likely to catch a cold (risk ratio 0.97), but it made colds 8% shorter in adults (Hemilä 2013). In 10 trials of at least 1 g a day, colds were 15% less severe (Hemilä 2023).

Grade B for starting it once a cold begins: 7 comparisons showed no consistent effect. Grade B for people under heavy physical stress: in 5 trials of people under heavy physical strain (598 in all, from marathons, skiing and subarctic army exercises), the risk of a cold halved (risk ratio 0.48) (Hemilä 2013). Our database page grades vitamin C D for colds, from one trial; the Cochrane review is the better guide.

Does honey help a cough from a cold?

Grade C. A 2021 review of 14 studies in nose and throat infections found people coughed less often and less severely on honey than with usual care; against a placebo, in just 2 studies, the difference was unclear (Abuelgasim 2021). No honey for babies under 1 year: the CDC warns it can give them botulism (CDC).

Do probiotics help you catch fewer colds?

Grade B. In a 2022 Cochrane review, probiotics (live bacteria in some yogurts and capsules) left about a quarter fewer people with an upper airway infection (risk ratio 0.76, 16 trials, 4,798 people). Infections were about a day shorter and fewer people needed antibiotics, but the evidence was mostly low certainty and the strains varied (Zhao 2022).

Does echinacea work?

Grade C for catching fewer colds. In the Cochrane review (24 trials, 4,631 people), none of the 12 prevention comparisons found a clear effect alone; pooled after the fact, they hinted at a 10% to 20% lower risk (Karsch-Völk 2014). A 2019 analysis found a risk ratio of 0.78 but said it is debatable whether that matters clinically, and the products tested varied widely (David 2019).

Grade B for no help once a cold has started: only 1 of 7 treatment trials found shorter colds (Karsch-Völk 2014), and the 2019 analysis saw no clear change in duration (David 2019).

What about garlic, pelargonium and NAC?

Grade D for garlic. The Cochrane review found a single eligible trial: 146 people took a garlic supplement or placebo for 12 weeks in winter, with 24 colds against 65. Once a cold started, recovery took about as long (Lissiman 2014).

Grade C for pelargonium (umckaloabo), a South African geranium root. In one trial, 105 adults with a cold took root-extract tablets or placebo for 10 days; by day 10, 45% against 12% had no symptoms left (Riley 2019). Cochrane reviewers rated the cold evidence very low quality and suspected publication bias (Timmer 2013).

Grade C for NAC (N-acetylcysteine). In one trial of 262 people, mostly over 65, NAC tablets twice a day for 6 winter months led to fewer and milder flu-like illnesses. As many people caught the flu virus, but fewer got sick from it: 25% of infected people on NAC against 79% on placebo (De Flora 1997). It has not been repeated.

What else helps?

Grade A for the flu vaccine in healthy adults: across trials of 71,221 people, it cut confirmed flu in a season from 2.3% to 0.9%, so 71 people need the shot to spare one a case (Demicheli 2018). Grade B in people 65 and over: 6% against 2.4%, from older, low-certainty trials (Demicheli 2018). The CDC advises a flu vaccine every season for everyone 6 months and older, ideally by the end of October (CDC).

Grade B for handwashing. In 9 trials of 52,105 people, mostly at schools, childcare centres and homes, hand hygiene programs cut the number of people with respiratory infections by 14%, though they made little or no difference to confirmed flu (Jefferson 2023).

Grade C for sleep. In 164 healthy adults whose sleep was tracked with a wrist monitor before cold-virus drops were put in their noses, those sleeping under 6 hours a night had about four times the odds of a cold of those sleeping more than 7 (odds ratios 4.50 and 4.24) (Prather 2015).

What we'd skip

  • Zinc nasal gels and sprays. They caused lost smell in case series (Alexander 2006; Davidson 2010).
  • Daily zinc all winter to avoid colds. Little or no effect in prevention trials (Nault 2024), and months of high doses can drain copper.
  • Vitamin D megadoses to ward off colds. No clear protection in 40 trials (Jolliffe 2025); large single doses showed no benefit even in the 2017 analysis (Martineau 2017).
  • High-dose vitamin C started once you're sick. No consistent effect in the trials that tested it (Hemilä 2013).
  • Echinacea to shorten a cold you already have. Treatment trials mostly found no difference (Karsch-Völk 2014).
  • Homemade elderberry from raw berries, leaves or stems. They can cause vomiting and severe diarrhea (NCCIH).

Safety and interactions

  • Zinc: when used for a cold, minor side effects (the review counted things like an unpleasant taste and stomach cramps) were more common than on placebo (risk ratio 1.34) (Nault 2024). Lozenge trials mostly gave 80 to 92 mg a day (some about 200 mg), well above the adult upper limit of 40 mg, but only for days to 3 weeks (Hemilä 2017); 50 mg or more a day for weeks can lower your copper (NIH ODS). Low copper causes anemia and nerve damage: among 70 people prescribed zinc, 9% developed unexplained anemia and 7% nerve symptoms (Duncan 2015), and in drug safety reports it showed up after a median of 137 days (Hiyama 2026). Zinc blocks the absorption of quinolone and tetracycline antibiotics and penicillamine, so take them apart (NIH ODS).
  • Vitamin D: the adult upper limit is 4,000 IU a day. Too much raises blood calcium, causing nausea, vomiting, weakness and kidney stones. It interacts with thiazide water pills (risk of high calcium), steroids, statins and the weight-loss drug orlistat (NIH ODS).
  • Vitamin C: the adult upper limit is 2,000 mg a day; high doses cause diarrhea, nausea and cramps (NIH ODS). In men, 1,000 mg a day or more from supplements was linked to 19% more kidney stones; not in women (Ferraro 2016).
  • Elderberry: raw or unripe berries, leaves and stems can cause nausea, vomiting and severe diarrhea (NCCIH); the CDC once reported a group poisoned by homemade elderberry juice (CDC 1984). No interaction studies were found.
  • Echinacea: allergic reactions, which can be severe, especially if you react to daisies or ragweed (its relatives), plus stomach upset (NCCIH). In a trial in children, rash was more common on echinacea (7.1% against 2.7% of colds) (Taylor 2003). Whether it interacts with drugs broken down by the liver is unclear (NCCIH).
  • Garlic: the trial reported rash and odor (Lissiman 2014). Case reports describe bleeding with garlic; a review found no clear evidence of a warfarin interaction (Vaes 2000). If you take a blood thinner or have surgery coming up, ask first.
  • Probiotics: side effects were minor, mostly gut symptoms (Zhao 2022). Serious infections, often in the blood, are rare but real: a review of case reports found 93 patients, half of them infected by a yeast probiotic (Saccharomyces) (Costa 2018). If you are very ill, have a weak immune system or an IV line, ask first.
  • Pelargonium: side effects were more frequent than on placebo, none serious (Timmer 2013). Of 13 reported liver-injury cases, a causality review could link none to it (Teschke 2012); in a Berlin hospital study of 198 liver-injury cases, one was rated a possible link (Douros 2016). If your eyes or skin turn yellow or your urine darkens, stop it and get checked.
  • NAC: taken with nitroglycerin patches for angina, it caused side effects in 35%, mostly severe headaches (Ardissino 1997). On nitrates, ask your prescriber before trying it.

When to see a doctor

  • Get help now for: shortness of breath, chest pain or pressure, new confusion, bluish lips, or hardly passing urine.
  • Flu symptoms if you're at higher risk (65 or older, pregnant, or living with asthma, diabetes, heart disease or a weak immune system): call early. Flu antiviral medicines work best when started within two days of the first symptoms (CDC).
  • Symptoms that ease and then come back worse, such as a returning fever, which can mean a sinus, ear or chest infection.
  • A cough past 3 weeks. Doctors call a cough under 3 weeks acute, usually from a viral infection, and look for other causes after that (Irwin 2018).

Don't stop a prescribed medicine on your own; ask the prescriber or pharmacist. If you are pregnant or breastfeeding, ask your doctor or midwife before taking any of these, and about the flu vaccine. For children, ask a clinician; this guide gives no children's amounts.

FAQ

What is the best supplement for a cold? Zinc acetate or gluconate lozenges, started within 24 hours of the first symptoms, have the best evidence for a shorter cold (grade B). Stop them when the cold is over.

Does vitamin D prevent colds and flu? Not clearly. A 2025 analysis of 40 trials found no significant protection (grade A). Treat a low level if a test shows one, for its own sake.

What actually lowers my chance of getting the flu? A yearly flu vaccine (grade A in healthy adults). Handwashing cuts colds and other respiratory infections (grade B), but trials have not clearly shown fewer confirmed flu cases. Short sleep is linked to catching colds (grade C); it has not been tested for flu.

Sources

  1. Nault D, et al. Zinc for prevention and treatment of the common cold. Cochrane Database Syst Rev. 2024. PubMed 38719213 · doi:10.1002/14651858.CD014914.pub2
  2. Hemilä H. Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open. 2017. PubMed 28515951 · doi:10.1177/2054270417694291
  3. Jolliffe DA, et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. Lancet Diabetes Endocrinol. 2025. PubMed 39993397 · doi:10.1016/S2213-8587(24)00348-6
  4. Hemilä H, et al. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013. PubMed 23440782 · doi:10.1002/14651858.CD000980.pub4
  5. Wieland LS, et al. Elderberry for prevention and treatment of viral respiratory illnesses: a systematic review. BMC Complement Med Ther. 2021. PubMed 33827515 · doi:10.1186/s12906-021-03283-5
  6. Macknin M, et al. Elderberry Extract Outpatient Influenza Treatment for Emergency Room Patients Ages 5 and Above: a Randomized, Double-Blind, Placebo-Controlled Trial. J Gen Intern Med. 2020. PubMed 32929634 · doi:10.1007/s11606-020-06170-w
  7. Zhao Y, et al. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. 2022. PubMed 36001877 · doi:10.1002/14651858.CD006895.pub4
  8. Demicheli V, et al. Vaccines for preventing influenza in healthy adults. Cochrane Database Syst Rev. 2018. PubMed 29388196 · doi:10.1002/14651858.CD001269.pub6
  9. Hemilä H, et al. Zinc acetate lozenges for treating the common cold: an individual patient data meta-analysis. Br J Clin Pharmacol. 2016. PubMed 27378206 · doi:10.1111/bcp.13057
  10. Hemilä H, et al. The effectiveness of high dose zinc acetate lozenges on various common cold symptoms: a meta-analysis. BMC Fam Pract. 2015. PubMed 25888289 · doi:10.1186/s12875-015-0237-6
  11. Hemilä H, et al. Zinc acetate lozenges for the treatment of the common cold: a randomised controlled trial. BMJ Open. 2020. PubMed 31980506 · doi:10.1136/bmjopen-2019-031662
  12. Alexander TH, et al. Intranasal zinc and anosmia: the zinc-induced anosmia syndrome. Laryngoscope. 2006. PubMed 16467707 · doi:10.1097/01.mlg.0000191549.17796.13
  13. Davidson TM, et al. The Bradford Hill criteria and zinc-induced anosmia: a causality analysis. Arch Otolaryngol Head Neck Surg. 2010. PubMed 20644061 · doi:10.1001/archoto.2010.111
  14. Autier P, et al. Vitamin D, acute respiratory infections, and Covid-19: The curse of small-size randomised trials. A critical review with meta-analysis of randomised trials. PLoS One. 2025. PubMed 39808630 · doi:10.1371/journal.pone.0303316
  15. Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017. PubMed 28202713 · doi:10.1136/bmj.i6583
  16. Jolliffe DA, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021. PubMed 33798465 · doi:10.1016/S2213-8587(21)00051-6
  17. Jolliffe DA, et al. Effect of a test-and-treat approach to vitamin D supplementation on risk of all cause acute respiratory tract infection and covid-19: phase 3 randomised controlled trial (CORONAVIT). BMJ. 2022. PubMed 36215226 · doi:10.1136/bmj-2022-071230
  18. Zakay-Rones Z, et al. Randomized study of the efficacy and safety of oral elderberry extract in the treatment of influenza A and B virus infections. J Int Med Res. 2004. PubMed 15080016 · doi:10.1177/147323000403200205
  19. Tiralongo E, et al. Elderberry Supplementation Reduces Cold Duration and Symptoms in Air-Travellers: A Randomized, Double-Blind Placebo-Controlled Clinical Trial. Nutrients. 2016. PubMed 27023596 · doi:10.3390/nu8040182
  20. Hemilä H, et al. Vitamin C reduces the severity of common colds: a meta-analysis. BMC Public Health. 2023. PubMed 38082300 · doi:10.1186/s12889-023-17229-8
  21. Abuelgasim H, et al. Effectiveness of honey for symptomatic relief in upper respiratory tract infections: a systematic review and meta-analysis. BMJ Evid Based Med. 2021. PubMed 32817011 · doi:10.1136/bmjebm-2020-111336
  22. Karsch-Völk M, et al. Echinacea for preventing and treating the common cold. Cochrane Database Syst Rev. 2014. PubMed 24554461 · doi:10.1002/14651858.CD000530.pub3
  23. David S, et al. Echinacea for the prevention and treatment of upper respiratory tract infections: A systematic review and meta-analysis. Complement Ther Med. 2019. PubMed 31126553 · doi:10.1016/j.ctim.2019.03.011
  24. Lissiman E, et al. Garlic for the common cold. Cochrane Database Syst Rev. 2014. PubMed 25386977 · doi:10.1002/14651858.CD006206.pub4
  25. Riley DS, et al. Treatment with (extract code omitted), a Pelargonium Sidoides Root Extract, Is Effective and Safe in Patients with the Common Cold: Results From a Randomized, Double Blind, Placebo-Controlled Clinical Trial. Integr Med (Encinitas). 2019. PubMed 31341433
  26. Timmer A, et al. Pelargonium sidoides extract for treating acute respiratory tract infections. Cochrane Database Syst Rev. 2013. PubMed 24146345 · doi:10.1002/14651858.CD006323.pub3
  27. De Flora S, et al. Attenuation of influenza-like symptomatology and improvement of cell-mediated immunity with long-term N-acetylcysteine treatment. Eur Respir J. 1997. PubMed 9230243 · doi:10.1183/09031936.97.10071535
  28. Demicheli V, et al. Vaccines for preventing influenza in the elderly. Cochrane Database Syst Rev. 2018. PubMed 29388197 · doi:10.1002/14651858.CD004876.pub4
  29. Jefferson T, et al. Physical interventions to interrupt or reduce the spread of respiratory viruses. Cochrane Database Syst Rev. 2023. PubMed 36715243 · doi:10.1002/14651858.CD006207.pub6
  30. Prather AA, et al. Behaviorally Assessed Sleep and Susceptibility to the Common Cold. Sleep. 2015. PubMed 26118561 · doi:10.5665/sleep.4968
  31. Duncan A, et al. The risk of copper deficiency in patients prescribed zinc supplements. J Clin Pathol. 2015. PubMed 26085547 · doi:10.1136/jclinpath-2014-202837
  32. Hiyama Y, et al. Characterizing clinical patterns and associated factors of zinc-induced copper deficiency: Insights from large-scale pharmacovigilance databases. Clin Nutr ESPEN. 2026. PubMed 42217623 · doi:10.1016/j.clnesp.2026.103368
  33. Ferraro PM, et al. Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones. Am J Kidney Dis. 2016. PubMed 26463139 · doi:10.1053/j.ajkd.2015.09.005
  34. Centers for Disease Control (CDC). Poisoning from elderberry juice--California. MMWR Morb Mortal Wkly Rep. 1984. PubMed 6422238
  35. Taylor JA, et al. Efficacy and safety of echinacea in treating upper respiratory tract infections in children: a randomized controlled trial. JAMA. 2003. PubMed 14657066 · doi:10.1001/jama.290.21.2824
  36. Vaes LP, et al. Interactions of warfarin with garlic, ginger, ginkgo, or ginseng: nature of the evidence. Ann Pharmacother. 2000. PubMed 11144706 · doi:10.1345/aph.10031
  37. Costa RL, et al. Infectious complications following probiotic ingestion: a potentially underestimated problem? A systematic review of reports and case series. BMC Complement Altern Med. 2018. PubMed 30541524 · doi:10.1186/s12906-018-2394-3
  38. Teschke R, et al. Initially purported hepatotoxicity by Pelargonium sidoides: the dilemma of pharmacovigilance and proposals for improvement. Ann Hepatol. 2012. PubMed 22700632
  39. Ardissino D, et al. Effect of transdermal nitroglycerin or N-acetylcysteine, or both, in the long-term treatment of unstable angina pectoris. J Am Coll Cardiol. 1997. PubMed 9120179 · doi:10.1016/s0735-1097(97)00005-3
  40. Irwin RS, et al. Classification of Cough as a Symptom in Adults and Management Algorithms: CHEST Guideline and Expert Panel Report. Chest. 2018. PubMed 29080708 · doi:10.1016/j.chest.2017.10.016
  41. Douros A, et al. Herb-Induced Liver Injury in the Berlin Case-Control Surveillance Study. Int J Mol Sci. 2016. PubMed 26784183 · doi:10.3390/ijms17010114