BACKGROUND AND PURPOSE:

• Zinc supplements are commonly taken to prevent or shorten colds, but evidence to support this practice is generally lacking
• Nault et al. (Cochrane Database of Systematic Reviews, 2024) assessed the effectiveness and safety of zinc for the prevention and treatment of the common cold

METHODS:

• Systematic review and meta-analysis
• Inclusion criteria
  • RCTs
  • Studies that tested any form of zinc against placebo to prevent or treat the common cold or upper respiratory infection (URTI) in children or adults
• Study design
  • Risk of bias was assessed with the Cochrane risk of bias tool
  • Certainty of evidence was assessed using GRADE Criteria
• Primary outcomes
  • Proportion of participants developing colds
  • Duration of cold | Measured in days from start to resolution of the cold
  • Adverse events potentially due to zinc supplements
    • e.g., unpleasant taste, loss of smell, vomiting, stomach cramps, and diarrhea
  • Adverse events considered to be potential complications of the common cold
    • e.g. respiratory bacterial infections

RESULTS:

• 15 prevention studies | 19 treatment studies | 8526 participants
  • On adults: 22 studies | On children: 12 studies
• Most studies evaluated the effectiveness of zinc administered as lozenges
  • Zinc gluconate doses: range 45 and 276 mg/day
  • Time period: range 4.5 and 21 days
• Other forms of zinc administration: Capsules | Dissolved powders | Tablets | Syrups | Intranasal sprays
• Most studies were at unclear or high risk of bias in at least one domain

Prevention of Colds

• There may be little or no reduction in the risk of developing a cold with zinc compared to placebo
  • Risk ratio (RR) 0.93 (95% CI, 0.85 to 1.01)
  • I2=20% | 9 studies | 1449 participants | Low-certainty evidence
• There may be little or no reduction in the mean number of colds that occur over five to 18 months of follow‐up
  • Mean difference (MD) –0.90 (95% CI –1.93 to 0.12)
  • I2 =96% | 2 studies | 1284 participants | Low‐certainty evidence
• If a cold does occur despite use of zinc
  • There is probably little or no difference in the duration of colds in days
    • MD –0.63 (95% CI –1.29 to 0.04)
    • I2 =77% | 3 studies | 740 participants | Moderate‐certainty evidence
  • There may be little or no difference in global symptom severity
    • Standardized mean difference 0.04 (95% CI, –0.35 to 0.43)
    • I2 =0% | 2 studies | 101 participants | Low‐certainty evidence

Used for Cold Treatment

• There may be a reduction in the mean duration of the cold in days
  • MD –2.37 (95% CI, –4.21 to –0.53)
  • I2 =97% | 8 studies | 972 participants | Low‐certainty evidence
• It is uncertain whether there is a reduction in the risk of
  • Having an ongoing cold at the end of follow‐up
    • RR 0.52 (95% CI, 0.21 to 1.27)
    • I2 =65% | 5 studies | 357 participants | Very low‐certainty evidence
  • Global symptom severity
    • SMD ‐0.03 (95% CI, –0.56 to 0.50)
    • I2 =78% | 2 studies | 261 participants | Very low‐certainty evidence

Adverse events

• 31 studies | 2422 participants
• It is uncertain whether there is a
  • Difference in the risk of adverse events with zinc used for cold prevention
    • RR 1.11 (95% CI, 0.84 to 1.47)
    • I2 =0% | 7 studies | 1517 participants | Very low‐certainty evidence
  • Increase in the risk of serious adverse events
    • RR 1.67 (95% CI, 0.78 to 3.57)
    • I2 =0% | 3 studies | 1563 participants | Low‐certainty evidence
• There is probably an increase in the risk of non‐serious adverse events when zinc is used for cold treatment
  • RR 1.34 (95% CI, 1.15 to 1.55)
  • I2 =44% | 2084 participants | 16 studies | Moderate‐certainty evidence
• No treatment study provided information on serious adverse events
• No study provided clear information about adverse events considered to be potential complications of the common cold

CONCLUSION:

• Zinc supplements may have little to no effect on common cold prevention, but may reduce the duration of ongoing colds
• The authors state

Overall, there was wide variation in interventions (including concomitant therapy) and outcomes across the studies, as well as incomplete reporting of several domains, which should be considered when making conclusions about the efficacy of zinc for the common cold

Learn More – Primary Sources:

Zinc for prevention and treatment of the common cold