The most effective things you can take to prevent getting sick are annual vaccines, vitamin D, zinc, and specific probiotics, though each works differently and some have stronger evidence than others. No single supplement will make you invincible, but a handful of well-studied options can meaningfully reduce how often you catch respiratory infections and how long they last when you do.
Vaccines Are Still the Strongest Defense
Nothing you can swallow as a supplement comes close to the protection offered by staying current on your flu and COVID-19 vaccines. The CDC’s 2025-2026 guidance recommends a COVID-19 vaccine for everyone six months and older, with the strongest recommendation for adults 65 and older and people with underlying health conditions. Adults 65 and older are now advised to get two doses spaced six months apart rather than one. For ages 12 through 64, the decision is framed around individual risk: the benefit is greatest if you have conditions like diabetes, obesity, or lung disease.
Flu vaccination follows a similar annual schedule each fall. Together, these two vaccines target the respiratory viruses most likely to put you in bed for a week or land you in a hospital. Everything below is supplemental, not a replacement.
Vitamin D: The Baseline to Get Right
Vitamin D plays a direct role in activating immune cells that fight off respiratory pathogens. The problem is that a large portion of the population runs low, especially during winter months when sun exposure drops. The NIH considers blood levels of 50 nmol/L (20 ng/mL) or above sufficient for most people, while levels below 30 nmol/L (12 ng/mL) raise your risk of deficiency. The zone between 12 and 20 ng/mL is a gray area where some people may not be getting enough.
If you’ve never had your vitamin D checked, a simple blood test can tell you where you stand. Most adults in northern climates benefit from 1,000 to 2,000 IU daily during the fall and winter. Don’t go overboard: levels above 125 nmol/L (50 ng/mL) can cause adverse effects including calcium buildup.
Vitamin C Works Best Under Physical Stress
Vitamin C’s reputation as a cold-fighter is more nuanced than most people realize. In the general population, regular supplementation does not reduce the number of colds you get. It can modestly shorten how long a cold lasts, but the effect is small for people with average activity levels.
The picture changes dramatically for people under heavy physical stress. In five trials involving 598 athletes, soldiers, and marathon runners, vitamin C cut cold risk by 52%. One large trial also found that people taking vitamin C experienced roughly 30% fewer total days of disability (days stuck at home or off work) compared to placebo. If you exercise intensely, train for endurance events, or work in physically demanding conditions, a daily dose of 200 to 1,000 mg has solid evidence behind it. For everyone else, it’s a nice-to-have rather than a game changer.
Zinc Lozenges: Dose and Form Matter
Zinc can shorten a cold significantly, but only if you get the dose and form right. A systematic review found that every trial using less than 75 mg of elemental zinc per day showed zero benefit. Seven out of eight trials using more than 75 mg per day showed a clear effect. Zinc acetate lozenges performed best, reducing cold duration by 42%. Other zinc salts at the same dose reduced it by about 20%.
This is where things get tricky for prevention. Those studies measured treatment (taking zinc after symptoms start), not daily supplementation. The daily upper limit for zinc in adults is 40 mg, and taking 50 mg or more for weeks at a time can interfere with copper absorption, weaken your immune function, and lower good cholesterol. So the smart approach is to keep zinc lozenges on hand and start them at the first sign of a scratchy throat, aiming for above 75 mg of elemental zinc per day for a few days, not as an ongoing daily habit.
Probiotics for Fewer Respiratory Infections
Your gut microbiome communicates constantly with your immune system, and specific probiotic strains have been shown in clinical trials to reduce how often people catch colds and flu-like illnesses. The best-studied strain is Lactobacillus rhamnosus GG, which has been shown to reduce rhinovirus infection in adults and lower the overall incidence of respiratory infections in children. Other strains with clinical evidence include L. casei Shirota (reduced incidence and duration of upper respiratory infections in working adults), L. plantarum L-137 (fewer infections in people under high psychological stress), and L. paracasei N1115 (stronger resistance to respiratory infections in older adults).
Combination products also show promise. A mix of L. rhamnosus GG with B. breve 99 and other strains reduced respiratory infections in children, while a combination of L. paracasei, L. casei 431, and L. fermentum PCC strengthened resistance to cold and flu-like infections in adults. When shopping for a probiotic, look for products that list specific strains (not just the species) on the label, because efficacy is strain-specific.
Elderberry Blocks Viruses From Entering Cells
Elderberry extract contains flavonoids that physically block the surface proteins influenza viruses use to latch onto your cells. When those surface proteins are deactivated, the virus can’t attach to the cell wall, enter, or replicate. Lab studies have confirmed this mechanism works against several influenza strains.
The clinical evidence is promising but smaller in scale than what exists for zinc or vitamin D. Most positive trials used standardized elderberry extracts taken at the onset of symptoms rather than daily for prevention. If you want elderberry in your toolkit, it makes the most sense as something you start at the first sign of illness, similar to zinc lozenges. Look for a standardized extract rather than homemade syrups, where the concentration of active compounds is unpredictable.
Echinacea: Modest but Real Benefits
Echinacea purpurea is one of the most popular herbal remedies for colds, and meta-analyses support a modest effect. Pooled data show it reduces the incidence of upper respiratory infections by about 19% and cuts antibiotic use significantly. It also appears to shorten how long infections last, though the effect size is small.
Where echinacea gets more interesting is in recurrence. People prone to repeated infections, particularly children who develop frequent ear infections, saw a meaningful reduction in episodes. The herb seems to work better as a preventive taken regularly during cold season than as an acute treatment started after symptoms are already in full swing.
NAC Supports Lung and Mucus Defenses
N-acetylcysteine (NAC) is a compound your body uses to produce glutathione, its primary internal antioxidant. Glutathione levels drop when you’re fighting an infection, and NAC supplementation helps restore them. NAC also breaks down the chemical bonds in mucus, making it thinner and easier to clear from your airways. This dual action, boosting antioxidant defenses while keeping mucus from becoming thick and stagnant, makes it particularly useful for people prone to bronchitis, sinus infections, or lingering chest colds.
By reducing mucus viscosity, NAC helps prevent the bacterial buildup that often turns a simple viral cold into a secondary infection. It also lowers markers of inflammation in the airways. Most studies have used doses between 600 and 1,200 mg daily. NAC is especially worth considering if you have a history of respiratory issues or if you smoke, since both situations deplete glutathione faster than normal.
Putting It All Together
A practical prevention strategy layers these tools based on timing. Daily, year-round habits include maintaining adequate vitamin D levels, taking a probiotic with clinically studied strains, and staying current on vaccines. During cold and flu season, adding echinacea and vitamin C makes sense, particularly if you exercise hard or face high stress. Then, at the very first sign of illness, zinc acetate lozenges (above 75 mg elemental zinc per day, for just a few days) and elderberry extract give you the best shot at cutting the infection short before it takes hold.