Do zinc lozenges shorten a cold? What the trials show, and the catch

Zinc lozenges may modestly shorten a common cold — by roughly a day or two — if they are taken at high doses within the first day of symptoms [s1][s2][s3]. But the evidence rests on a small number of trials, the effect depends heavily on the lozenge's formulation and dose, and taking zinc to prevent colds has much weaker support [s1][s2].

What "shortens a cold" actually means here

The clearest treatment signal comes from meta-analyses of lozenge trials. A 2017 meta-analysis in JRSM Open pooled seven randomised, placebo-controlled trials with 575 participants, restricted to trials that used more than 75 mg of zinc a day [s2]. Across them, cold duration was 33% shorter in the zinc groups than in placebo groups (95% confidence interval 21% to 45%) [s2]. An individual-patient-data meta-analysis of three zinc acetate trials, covering 199 patients, put the absolute effect at about 2.73 to 2.94 days shorter, against an average cold length of seven days in those trials [s3].

That is a real but bounded effect: a cold cut from about a week to about five days, in people who started lozenges promptly and used a lot of zinc.

Formulation and dose: where the claims get muddy

The 2017 analysis is useful precisely because it tested two things marketers often blur. First, the salt: trials using zinc acetate lozenges found colds shortened by 40%, and those using zinc gluconate by 28% — but the 12-percentage-point gap between them was not statistically significant (95% CI −12 to +36), so there is no firm evidence one salt beats the other [s2]. Second, the dose: five trials using 80–92 mg/day produced a 33% reduction, and two trials using 192–207 mg/day produced 35%, a difference that was again not significant [s2]. The authors concluded there is no evidence that pushing zinc above roughly 100 mg/day improves the effect [s2].

The practical reading is that more is not better, and the specific brand's chemistry may matter less than getting an adequate dose early — but the trial base is too small to be prescriptive, which is why this remains an informational summary rather than a dosing guide.

Prevention is a weaker case

Taking zinc to stop colds happening is a different claim, and it holds up less well. A 2021 rapid review in BMJ Open pooled 28 randomised trials with 5,446 participants across prevention and treatment of viral respiratory infections in adults [s1]. It found oral or intranasal zinc prevented about 5 respiratory infections per 100 person-months versus placebo (95% CI 1 to 8), a number-needed-to-treat of 20 [s1]. But sublingual zinc did not prevent colds when volunteers were deliberately inoculated with rhinovirus (relative risk 0.96, 95% CI 0.77 to 1.21) [s1].

On treatment, the same review found symptoms resolved on average two days earlier with sublingual or intranasal zinc than with placebo, and that 19 more adults per 100 were still symptomatic on day 7 without zinc [s1]. It also flagged the softer edge of the data: day-3 symptom severity fell, but average daily symptom severity across the illness did not differ significantly (standardised mean difference −0.15, 95% CI −0.43 to 0.13), and it graded much of the treatment evidence as low or very low certainty [s1].

The trade-offs

Zinc is not free of downsides. The BMJ Open review found non-serious adverse events — nausea, and mouth or nasal irritation — were more common with zinc than placebo (relative risk 1.41, 95% CI 1.17 to 1.69), enough that tolerability may be a limiting factor for some people [s1]. Reassuringly, no serious adverse events were reported in the 25 trials that monitored for them [s1].

A further honest caveat: much of the lozenge-duration evidence comes from a small circle of trials and overlapping meta-analyses, and the studies vary in quality [s2][s3]. That is a thinner foundation than the confident packaging on a lozenge tin implies.

Bottom line

The claim survives, narrowed. High-dose zinc lozenges, started at the first sign of a cold, can shorten it by around a third — a day or two in practice — though the evidence base is small and the effect on overall symptom severity is uncertain [s1][s2][s3]. Prevention claims are weaker, and side effects are common if not serious [s1]. As with other over-the-counter remedies whose reputations outrun their trials — a pattern also visible in the evidence on probiotics and on vitamin D in healthy adults — the useful version of the advice is modest, specific, and not a substitute for a clinician's judgement in anyone who is unwell.

Sources

• Zinc for the prevention or treatment of acute viral respiratory tract infections in adults: a rapid systematic review and meta-analysis of randomised controlled trials — BMJ Open, 2021-11-02
• Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage — JRSM Open, 2017-05-02
• Zinc acetate lozenges for treating the common cold: an individual patient data meta-analysis — British Journal of Clinical Pharmacology, 2016-07-05