Conclusion
Zinc may shorten a cold slightly, but only under specific conditions. Reductions in duration have been seen when zinc lozenges are dissolved slowly in the mouth, started within 24 hours of the first symptoms, and taken at more than 75 mg per day.
Zinc has not been shown to prevent colds, and results across studies vary widely, so the certainty is low.
How much does it help?
A 2024 systematic review reported colds shortened by about 2 days on average, though variation between trials was very high. Analyses focused specifically on lozenges suggest roughly a one-third reduction in duration, with the largest effect from zinc acetate. Swallowed tablets and syrups show little clear benefit.
What the research shows
The evidence includes a 2024 systematic review of 34 trials (about 8,500 participants) and several meta-analyses focused on lozenges. Studies differ considerably in the form of zinc used (acetate, gluconate), the dose, and the delivery method.
This field also has a history of a review being withdrawn over errors and improper citation, so the conclusions are not settled.
Cautions
- The findings apply to lozenges dissolved in the mouth, not swallowed tablets; a different method is unlikely to give the same result
- High-dose zinc can cause an unpleasant taste, nausea, and stomach upset, so short courses are more sensible
- Nasal zinc products have been linked to loss of smell and are best avoided
- Any benefit appears at the very start of symptoms; there is little support for starting once a cold is established
A simple analogy
A zinc lozenge is like throwing water on a fire while it is still small. Caught at the very first spark, it may keep things from spreading, but once the fire is large the difference fades. And if the water is thrown the wrong way (the wrong form, or not dissolved slowly), it does not work the same.