Zinc made about a third more people feel unwell than placebo did
Across 16 trials and 2,084 people, non-serious side effects came about 34% more often on zinc. That figure comes from a week of high-dose lozenges during a cold.
What the study found
About a third more people felt worse on zinc than placebo
The clearest result in the cold literature is not a benefit. Across 16 trials and 2,084 people, non-serious side effects came about 34% more often on zinc than on placebo, and that carries more certainty than the shortened cold does. No treatment trial recorded a serious adverse event.
The caveat · A week of lozenges, not daily pills
The figure comes from roughly a week of high-dose lozenges taken during a cold, not from a daily supplement taken for months. It counts non-serious events only, and no treatment study recorded a serious one. Sixteen trials contributed, most at unclear or high risk of bias in at least one area.