Probiotics (URTI strains)
Lactobacillus rhamnosus GG, L. casei Shirota, or L. paracasei/L. plantarum blends; 1–10 billion CFU/day, refrigerated or shelf-stable with CFU guaranteed at expiry, not at manufacture.
Moderate
Some good studies, not many repeats
Two independent meta-analyses (23 RCTs / 6,950 people; 20 RCTs) agree in direction and magnitude, and the antibiotic-use outcome is moderate-certainty GRADE. Held back from strong because most URTI outcomes are low-certainty, strains and doses are heterogeneous, and a well-run 20-week athlete trial (n=243) was flatly null.
It cuts your chance of catching a cold by about a quarter and makes each one about a day shorter. It takes 8 to 12 weeks of daily use to build up.
Cuts the chance of having at least one URTI by 24% (RR 0.76, 95% CI 0.67–0.87) and shortens each episode by about 1.2 days (95% CI 0.33–2.12 days shorter).
Colonises gut mucosa, raises secretory IgA and modulates dendritic-cell and NK activity along the gut–lung axis, so viral load is contained earlier and the systemic inflammatory response is smaller.
People with 3+ URTIs per season, high-exposure settings (childcare, classrooms, clinics, crowded transit), and heavy training loads. Benefit is largest in frequent-infection profiles — the ≥3-episode outcome had the strongest effect.
What to buy
- ✓Lactobacillus rhamnosus GG, L. casei Shirota, or L. paracasei/L. plantarum blends; 1–10 billion CFU/day, refrigerated or shelf-stable with CFU guaranteed at expiry, not at manufacture.
- ✓1–10 billion CFU/day, single daily dose, taken continuously for at least 8–12 weeks — the effect is preventive and builds over time, not acute.
- ✓Third-party tested (USP, NSF, Informed Sport)
- ✕Proprietary blends that hide the dose per ingredient
- ✕Unstandardised extracts with no active-compound percentage
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Lactobacillus rhamnosus GG — the exact strain the respiratory-infection trials used, not a generic multi-strain. Strain identity is the whole game with probiotics.
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Same LGG strain with a prebiotic added. Take it daily for at least eight to twelve weeks — the effect is preventive and builds, it does nothing for a cold you already have.
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The most-reviewed probiotic on Amazon with a hundred and forty thousand ratings — but be clear about the trade: it is a ten-strain blend, not the LGG the respiratory trials tested.
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See all on AmazonEvery result, next to the line that means no effect
Each diamond is a study's best estimate. The thin line shows how sure the study was: the shorter the line, the more certain the result.
- Risk of ≥1 URTI episode% relative risk · Cochrane 2022 (Zhao), 16 studies, n=4,798−24.0−33 … −13
- Antibiotic courses prescribed for URTI% relative risk · Cochrane 2022 (Zhao), 6 studies, n=1,548−42.0−58 … −19
- Mean episode durationdays · Cochrane 2022 (Zhao), 6 studies, n=2,406−1.22−2.12 … −0.33
- Illness duration (independent meta-analysis)days · King 2014, Br J Nutr, 20 RCTs−0.77−1.50 … −0.04
Each unit gets its own scale, so numbers are never mixed on one line. When a study gave no range, we leave it blank instead of inventing one.
3 studies over 8 years
The studies
- 1.Probiotics for preventing acute upper respiratory tract infectionsCochrane Database of Systematic Reviews · 2022 · rct · 23 experiments · n = 6,950
RR 0.76 (95% CI 0.67–0.87) for ≥1 URTI; RR 0.59 (0.38–0.91) for ≥3 URTIs; duration −1.22 days (−2.12 to −0.33); antibiotic use RR 0.58 (0.42–0.81). Adverse events no different from placebo (RR 1.02, 0.90–1.15).
- 2.Effectiveness of probiotics on the duration of illness in healthy children and adults who develop common acute respiratory infectious conditions: a systematic review and meta-analysisBritish Journal of Nutrition · 2014 · rct · 20 experiments
Illness shortened by 0.77 days (95% CI −1.50 to −0.04); fewer illness days per person (SMD −0.31, −0.41 to −0.11) and fewer days absent from work or school (SMD −0.17, −0.31 to −0.03).
- 3.Effects of Lactobacillus casei Shirota ingestion on common cold infection and herpes virus antibodies in endurance athletes: a placebo-controlled, randomized trialEuropean Journal of Applied Physiology · 2016 · rct · 1 experiments · n = 243
Null for clinical URTI: 48% vs 42% episode incidence (p=0.365), no difference in duration or severity. Did reduce plasma CMV and EBV antibody titres in seropositive athletes (p<0.01).
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