Does zinc supplementation shorten the duration of viral illnesses in adults?

Zinc Supplementation for Viral Respiratory Illnesses

Zinc acetate or zinc gluconate lozenges at doses ≥75 mg/day, started within 24 hours of symptom onset and continued throughout the illness, significantly reduce the duration of the common cold by approximately 2-2.4 days in adults. 1, 2

Evidence for Treatment of Active Viral Illness

The American Academy of Otolaryngology-Head and Neck Surgery recommends zinc lozenges specifically for shortening cold duration when used at therapeutic doses. 1, 2

Key Treatment Parameters

• Formulation matters: Use zinc acetate or zinc gluconate lozenges specifically—not tablets, capsules, or syrups 1
• Dose threshold: Must be ≥75 mg/day of elemental zinc to achieve benefit 1, 2
• Timing is critical: Must start within 24 hours of symptom onset 1, 2
• Duration of therapy: Continue throughout the entire cold, not just the first few days 1, 2

Magnitude of Benefit

The most recent high-quality systematic review (2024 Cochrane) found that when zinc is used for cold treatment, there may be a reduction in mean cold duration of 2.37 days (95% CI: -4.21 to -0.53 days), though the certainty of evidence is low due to significant heterogeneity between studies 3. An independent 2020 systematic review specifically examining zinc supplementation in healthy adults found a reduction of 2.25 days (95% CI: -3.39, -1.12) 4.

Evidence for Prevention

Zinc supplementation does NOT prevent colds from occurring. 1, 2, 3

• The American College of Physicians states that prophylactic zinc supplementation cannot be recommended due to insufficient data 2
• The 2024 Cochrane review found little or no reduction in the risk of developing a cold (RR 0.93,95% CI 0.85 to 1.01) 3
• There is no reduction in the mean number of colds over 5-18 months of follow-up 3

Safety Profile and Adverse Events

Zinc lozenges at therapeutic doses probably increase the risk of non-serious adverse events by 34%. 3

• Common side effects include unpleasant taste, nausea, and mouth irritation 1, 3
• The 2024 Cochrane review found moderate-certainty evidence for increased non-serious adverse events (RR 1.34,95% CI 1.15 to 1.55) when zinc is used for treatment 3
• No increase in serious adverse events was demonstrated 3

Critical Safety Warning

Avoid intranasal zinc preparations—these have been associated with permanent loss of smell (anosmia). 1 The evidence base focuses on oral lozenges, not nasal formulations.

Mechanism of Action

Zinc appears to work through multiple antiviral mechanisms 5, 6:

• Ionic zinc has electrical affinity for ICAM-1 receptors on both rhinovirus structures and nasal epithelial cells, potentially blocking viral attachment 6
• Zinc is a critical component of approximately 10% of the human proteome and influences immune function across all cell types 5
• Zinc-dependent proteins are involved in anti-viral defense and immune regulation in the respiratory tract 7

Clinical Application Algorithm

1. Patient presents within 24 hours of cold symptom onset → Consider zinc lozenges 1, 2
2. Verify formulation: Zinc acetate or gluconate lozenges only 1
3. Verify dose: ≥75 mg/day elemental zinc 1, 2
4. Counsel patient: Continue throughout entire cold duration, expect possible bad taste/nausea 1, 3
5. Patient presents >24 hours after onset → Zinc likely less effective; focus on symptomatic management 1, 2

Important Caveats

• The evidence quality is limited: The 2024 Cochrane review rated the evidence for treatment benefit as "low certainty" due to wide variation in interventions, incomplete reporting, and significant heterogeneity (I² = 97%) 3
• Not all zinc formulations work: The benefit is specific to lozenges that dissolve slowly in the mouth, allowing local zinc ion release 1, 6
• Combination products may not work: Avoid zinc combined with citric acid or tartaric acid, as these bind zinc ions and reduce bioavailability 6
• Population-specific: Most evidence comes from otherwise healthy adults in developed countries 3, 4

What Zinc Does NOT Do

• Does not prevent colds from occurring 1, 2, 3
• Does not reduce symptom severity (only duration) 3
• Does not reduce risk of complications 3
• Should not replace evidence-based symptomatic treatments like NSAIDs for pain/fever or ipratropium for rhinorrhea 1, 2

Comparison to Vitamin C

Unlike zinc, vitamin C shows consistent effects on reducing both duration AND severity of colds in regular supplementation studies, though it also does not prevent colds in the general population 1, 2. Given vitamin C's low cost and safety profile, the American College of Physicians suggests individual patients may trial it to determine personal benefit 2.