Researchers looked at 107 studies involving over 100,000 adults to figure out which supplements work best at preventing colds and flu. They found that certain probiotics (good bacteria), a compound called catechin found in tea, and high-dose vitamin D showed the most promise. Probiotics also helped people feel better faster when they did get sick. The good news? None of these supplements caused harmful side effects. However, scientists say we need more direct comparisons between different supplements to know for sure which ones work best and how much you should take.

The Quick Take

  • What they studied: Which supplements—like vitamins, probiotics, and compounds from tea—actually help adults avoid catching colds, flu, and other respiratory infections
  • Who participated: Over 101,000 healthy adults across 107 different research studies. Most were regular people without serious immune problems
  • Key finding: Three supplements stood out: multi-strain probiotics (good bacteria) reduced infection risk by 10%, catechin from tea reduced it by 21%, and Bifidobacterium animalis (a specific probiotic) reduced it by 21%. For COVID-19 and flu specifically, high-dose vitamin D was most effective, cutting risk by 34%
  • What it means for you: If you’re looking to avoid getting sick, probiotics or tea-based supplements may help slightly, but they’re not a guarantee. Vitamin D might be especially useful during flu season. These aren’t replacements for handwashing and other prevention methods, and you should talk to your doctor before starting any supplement routine

The Research Details

Scientists conducted a ’network meta-analysis,’ which is like a super-study that combines results from many smaller studies to find patterns. They searched four major medical databases for all studies published through February 2025 that tested supplements for preventing respiratory infections in adults. They only included studies where people took supplements by mouth (not injections) and where the goal was prevention, not treatment of existing illness.

They looked at four main types of supplements: micronutrients (like vitamins and minerals), flavonoids (plant compounds), probiotics (beneficial bacteria), and synbiotics (probiotics plus their food source). The researchers then used special statistical methods to compare all these supplements against each other and against placebo (fake pills), even when different studies hadn’t directly tested them against each other.

They tracked several outcomes: whether people got sick at all, how long symptoms lasted, how severe symptoms were, and whether anyone experienced side effects. They also rated how confident they could be in their findings based on study quality and consistency.

This approach is powerful because instead of just looking at one supplement at a time, it lets researchers see which supplements work best compared to each other. This helps people make informed choices about what to take. The researchers also carefully checked the quality of evidence, so readers know which findings are more trustworthy than others

This is a high-quality study because it included over 100,000 people across many different studies, making the results more reliable. The researchers were transparent about their methods and registered their plan before starting. However, the studies they reviewed used different forms of supplements (pills, drinks, mouthwashes) and different doses, which made some comparisons less clear. The researchers were honest about this limitation

What the Results Show

For preventing any respiratory infection, three supplements worked best. Catechin (a compound from green and black tea) reduced infection risk by about 21% compared to placebo. Two types of probiotics also worked: Bifidobacterium animalis reduced risk by 21%, and multi-strain probiotics (combinations of different beneficial bacteria) reduced risk by 10%. These results came from high-quality or moderately-quality studies, meaning we can be fairly confident in them.

When researchers looked specifically at COVID-19 and flu prevention, high-dose vitamin D was the clear winner, cutting risk by about 34%. This was based on nine studies and considered moderately reliable evidence.

For people who did get sick, probiotics helped them feel better faster. Multi-strain probiotics shortened how long symptoms lasted by about one day, and also made symptoms less severe overall. Catechin also shortened symptom duration by about 2-3 days, though this was based on only two studies.

Importantly, none of the supplements increased the risk of side effects or adverse events compared to placebo, suggesting they’re generally safe when used as directed.

The researchers also looked at specific types of infections like common colds and upper respiratory infections, though the results for these were less clear-cut. Some supplements showed promise for these specific infections, but the evidence wasn’t as strong. The study also found that different supplements worked better for different outcomes—for example, vitamin D was best for flu prevention, while probiotics were best for shortening symptom duration

This study builds on decades of research into supplements and immune health. Previous studies had suggested probiotics and vitamin D might help, but this is one of the largest and most comprehensive comparisons of different supplements side-by-side. The findings generally support what smaller studies have suggested, but with more confidence because of the larger number of people studied

The biggest limitation is that most studies tested supplements against placebo (fake pills) rather than against each other directly. This means we can’t be 100% sure which supplement is truly best. Also, different studies used different forms and doses of supplements—some used pills, others used drinks or mouthwashes—which made comparisons trickier. The studies also varied in quality, and some were quite small. Finally, most studies were done in developed countries, so results might not apply everywhere in the world

The Bottom Line

Based on moderate to high-quality evidence: Multi-strain probiotics appear to be a reasonable choice for general respiratory infection prevention and for shortening symptom duration if you do get sick (moderate confidence). High-dose vitamin D may be particularly helpful during flu and COVID-19 season (moderate confidence). Catechin supplements or drinking more tea may help prevent infections, though evidence is somewhat limited (moderate confidence). These supplements appear safe with no increased side effects. However, they should complement—not replace—basic prevention like handwashing, vaccination, and healthy habits

Anyone interested in naturally supporting their immune health during cold and flu season could consider these supplements. They may be especially relevant for people who want alternatives to medications or who have frequent respiratory infections. However, people with certain medical conditions, those taking specific medications, pregnant or breastfeeding women, and children should talk to their doctor first. These supplements are not replacements for vaccines or medical treatment if you do get sick

Don’t expect overnight results. Most studies tracked people over several months during cold and flu season. You’d likely need to take supplements consistently for at least a few weeks to several months to see benefits. If you do get sick despite taking supplements, you might notice symptoms lasting about one day less than usual, which is modest but meaningful

Want to Apply This Research?

  • Track daily supplement intake (type, dose, time) and weekly illness episodes (did you get sick? how many days of symptoms?). Compare your illness frequency before and after starting supplements over a 3-month period
  • Set a daily reminder to take your chosen supplement at the same time each day. Log it in the app immediately after taking it to build consistency. Also log any cold or flu symptoms that develop, including start date and duration, to see if supplements are helping
  • Create a monthly summary view showing: days sick per month, average symptom duration, and supplement adherence percentage. Compare data across seasons to see if supplements made a difference. Share this data with your doctor to discuss whether to continue, change, or stop supplementation

This research summary is for educational purposes only and should not replace professional medical advice. While these supplements appear safe in studies, they may interact with medications or be inappropriate for certain health conditions. Before starting any new supplement regimen, especially if you have existing health conditions, take medications, are pregnant or breastfeeding, or have a weakened immune system, consult with your healthcare provider. This study shows supplements may help prevent infections, but they are not guaranteed to prevent illness and should be combined with other prevention methods like vaccination, handwashing, and healthy lifestyle habits. If you develop symptoms of respiratory infection, seek appropriate medical care.