Do zinc lozenges shorten colds? Same trials, two readings

Search "does zinc work for colds" and you get two confident camps. One says zinc cuts a cold short. The other says the science never panned out. Each can point to a respectable paper. Reading those papers side by side turns out to be more useful than picking a camp, because the disagreement isn't about the trials. It's about how they get combined.

What Cochrane found

The 2024 Cochrane review, by Nault and colleagues, pulled together 34 randomized trials with 8,526 participants: 15 on preventing colds and 19 on treating them, 22 in adults and 12 in children. Its headline numbers:

Prevention. Taking zinc ahead of time made little or no clear difference to whether people caught a cold (risk ratio 0.93, 95% CI 0.85 to 1.01, 9 studies, low-certainty evidence).

Treatment. Once a cold had started, "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)." There was no clear effect on overall symptom severity.

Side effects. There is "probably an increase in the risk of non‐serious adverse events when zinc is used for cold treatment" (risk ratio 1.34, 16 studies, moderate certainty). The trials reported things like nausea, taste changes and mouth irritation.

The bottom line in the review's implications for practice: "the current evidence is insufficient to provide firm conclusions or recommend zinc supplementation for the prevention or treatment of the common cold." The review also notes that most included studies were funded by organizations with commercial ties or didn't report funding.

Notice two things in that treatment number. A 2.37-day average reduction sounds meaningful. But the confidence interval runs from about half a day to more than four days, and the I² of 97% says the eight trials disagreed with each other enormously. When studies scatter that much, the pooled average describes none of them well.

What the critique found

In October 2024, Harri Hemilä (University of Helsinki) and Elizabeth Chalker (Australian National University) published a short critique in Frontiers in Medicine. Hemilä has published on zinc lozenges for well over a decade, and he is an author of one of the trials Cochrane included, so read him as an expert with a long-held position, not a neutral referee. The paper declares no commercial or financial conflicts. Its argument is worth following step by step, because each point is a general rule for reading any meta-analysis.

1. Days are the wrong ruler. Cochrane expressed the effect as a mean difference in days. Hemilä and Chalker point out that "Colds can last for 1 day or 3 weeks and over. Obviously, 1-day colds cannot be shortened by 2.37 days." A percentage change fits illness duration better, they argue, and a scale that fits the data gives tighter estimates.

2. Lozenges and nasal gels aren't the same treatment. The 2.37-day analysis pooled zinc lozenges with nasal zinc. In the critique's words, one included lozenge trial gave 190 mg of zinc a day while one nasal trial gave 0.046 mg, and "Pooling two trials with a 4,300-fold difference in the dose is not meaningful."

3. Adults and children were mixed. One lozenge trial was in children, and its result differed sharply from the adult trials.

4. A placebo-controlled trial was left out of this analysis. They note Cochrane excluded the Mossad 1996 trial from the duration analysis without explanation, while using it elsewhere in the review.

Restricting the analysis to zinc lozenges in adults, adding the Mossad trial back, and using the percentage scale, they "calculated that zinc lozenges shortened colds in adults by 37% (95% CI: 27%−46%; P = 10⁻⁹)." They found no evidence of benefit in children. That fits Hemilä's earlier 2017 meta-analysis in JRSM Open: seven trials, 575 participants, all using more than 75 mg of zinc a day, with colds 33% shorter on average (40% in the three zinc acetate trials, 28% in the four zinc gluconate trials, a difference that was not statistically significant).

So who's right?

Both papers are doing what they set out to do, and they answer different questions. Cochrane asked, roughly, "does taking zinc, in any of the forms people have tested, help with colds?" Pool everything from nasal gels to 190 mg lozenges, in adults and kids, and you get a noisy average with low certainty. Hemilä and Chalker asked a narrower question: "do zinc lozenges at the doses that were tested shorten colds in adults?" Narrow the question and the answer gets much clearer.

What I'd hold onto: the critique is a reanalysis by researchers who have argued this position for years, published as a commentary, and a 37% figure from five adult trials is still five trials. Cochrane's broad caution and the critique's narrow confidence can both be read as accurate descriptions of their own question. The NIH Office of Dietary Supplements lands roughly in between: it calls the trial results "inconsistent" but says that, overall, zinc lozenges or syrup "appears to reduce the duration, but not the severity" of cold symptoms when taken shortly after a cold starts.

The formulation problem

This is the part that matters to me as someone who formulates supplements, and it's the part almost nobody covering zinc mentions. The proposed mechanism is local: the lozenge dissolves slowly and releases zinc ions in the mouth and throat. If that's right, the total zinc on the label isn't the whole story. What matters is how much free zinc the lozenge actually releases.

The critique notes that some lozenges tested in trials "contained citric acid, tartaric acid, mannitol-sorbitol, or other substances that bind zinc so that it is not freely released." A product can carry a big zinc number and still release little of it. That is a chemistry point, and a plausible reason trials of different lozenges reached different answers. It's also a reason a negative trial of one lozenge doesn't settle the question for another.

One honest caveat on dose. The lozenge trials that showed benefit used more than 75 mg of zinc a day, usually for one to two weeks. The NIH lists the Tolerable Upper Intake Level for adults at 40 mg a day and warns that 50 mg or more "over a period of weeks" can inhibit copper absorption. Hemilä and Chalker argue that short courses at trial doses are unlikely to cause lasting harm. Those two statements aren't in conflict, but they're a reason this is a short-course, clinician-conversation question, not a daily habit.

How to read a zinc lozenge label

Look at the form. The trials with the strongest results used zinc acetate or zinc gluconate lozenges, dissolved slowly in the mouth, not swallowed tablets or nasal products.

Check the other ingredients. Citric acid and tartaric acid are flagged in the critique as zinc binders. Their presence doesn't prove a product won't work, but it's the first question I'd ask a manufacturer.

Timing matters. The trials started zinc early in a cold. Nothing here suggests zinc taken on day four does much.

Don't take it as prevention. On the Cochrane data, taking zinc ahead of time showed little or no clear effect on whether you catch a cold.

Mind the dose and the duration. Trial doses sit above the daily upper limit. That's a conversation for your clinician, especially if you take other medications.

Not medical advice. This is educational analysis, not a recommendation — a study is not a prescription. Talk to a qualified clinician before acting on anything you read here. Full disclaimer →

Disclosure, plainly: I founded and run Shroombiosis (a company I run), which formulates and sells supplements. That is a commercial stake in how supplement research gets read. Shroombiosis does not sell a zinc lozenge, and I have no relationship with any author, review group or brand discussed here. Nothing here is sponsored and no link earns a commission; here's the full policy. A recommendation with no stake at all: for performance nutrition, Die Tryin Co. is a fellow combat-veteran-owned brand I'm glad to point people to. I don't own it and earn nothing from the link.