Most people who reach for zinc at the first sign of a cold are asking the wrong question. "Does zinc work?" has a reasonably clear answer in the research. "When should I start, how much, in what form, and when should I stop?" - those questions are where the data get specific, and where most supplement labels go quiet.
What the evidence is actually testing
The bulk of the randomized controlled trial data on zinc and the common cold involves zinc lozenges taken during active upper respiratory infection - not preventive supplements, not capsules swallowed with water, and not the small doses found in most multivitamins. Trials showing no effect often used doses below the level that seems mechanically relevant. Trials showing benefit usually used zinc lozenges with 75 mg or more of elemental zinc per day, dissolved slowly in the mouth to maximize contact with nasal and throat tissue.
The distinction matters because zinc's proposed mechanism in acute infection is local, not systemic. Zinc ions released in the upper airways may interfere with how rhinovirus attaches to cell receptors in the nasal passages. A lozenge dissolving slowly in the mouth acts differently than a zinc capsule that delivers the mineral to the intestine.
The 24-hour window
The most consistent finding across systematic reviews is that timing relative to symptom onset is the main factor in whether zinc lozenges shorten a cold. A 2020 systematic review in Nutrients found that starting zinc within 24 hours of symptom onset cut cold duration more than starting at the 24-to-48-hour mark - about 1.42 days shorter in the pooled analysis. A systematic review and meta-analysis in CMAJ Open found that zinc shortened cold duration in adults (by about 1.65 days compared to placebo) when used early in the illness.
What does this mean? Keep zinc lozenges accessible - it matters more than any other supplement choice during an illness. Starting at hour 36 helps, but the data show less effect than starting within the first full day of symptoms.
Here's one clear line from research: trials showing benefits were mostly in adults. The same systematic reviews found no effect on cold duration in children, and high-dose zinc has a different safety profile in kids than in adults.
The dose question
Below 75 mg per day of elemental zinc, there's no evidence that it shortens colds. A meta-analysis in JRSM Open by Harri Hemilä found that five trials using low-dose zinc lozenges (below 75 mg per day) showed no effect on cold duration. Trials using 75 mg per day or more showed about 33% reduction in cold duration (95% CI: 21% to 45%). Doses above 100 mg per day didn't add more benefit than doses of 80 to 92 mg per day.
To get 75 mg or more of elemental zinc per day from lozenges, you need to take one lozenge roughly every two hours while awake. The protocol that worked in trials means dissolving each lozenge slowly in the mouth - not chewing - at this rate, and starting within 24 hours of the first symptom.
Formulation: why it matters more than the label suggests
Not all zinc lozenges work the same. The same JRSM Open meta-analysis compared zinc acetate against other zinc salts, mainly zinc gluconate, and found a big difference in results. Trials using zinc acetate at doses above 75 mg per day showed a 42% reduction in cold duration. Trials using other zinc salts at similar doses showed a 20% reduction.
Why? It comes down to free zinc ion release. Zinc acetate dissolves easily, releasing ionized zinc. Some lozenges contain citric acid or other compounds that bind zinc ions and reduce the free zinc that can interact with tissues in the mouth and throat. Certain flavoring agents may do the same thing. Check if a product has citric acid, tartaric acid, or sorbitol alongside the zinc - these ingredients can reduce effectiveness, which may explain why some trials showed no effect. The bigger point: formulation determines what reaches the target tissue. This applies across supplements, as this piece on collagen absorption shows.
When to stop
You need to know when to stop, not just when to start. Therapeutic doses of 75 to 92 mg of elemental zinc per day are about double the Tolerable Upper Intake Level (UL) set by the NIH Office of Dietary Supplements at 40 mg per day. This limit exists partly to prevent zinc from interfering with copper absorption at high sustained doses.
For an acute cold, you use it for a short time. Trials that showed benefit typically had symptoms clear within five to seven days after starting zinc, compared to seven to ten days for placebo groups. The evidence covers this window only - it doesn't support continuing high-dose lozenges after symptoms resolve.
Copper problems start when people take therapeutic zinc doses for weeks or months - assuming that if zinc helped during this cold, it might prevent the next one. Studies show that daily intakes of 50 mg or more of zinc, sustained over time, reduce copper absorption and lower HDL cholesterol. At very high sustained doses - 150 mg per day for long periods - case reports show copper deficiency with neurological problems. Even moderately high doses sustained over time can cause low copper levels without obvious early symptoms.
If you switch from treating an acute cold to longer-term immune support, change your dose and form. The recommended dietary allowance for zinc is 11 mg per day for adult men and 8 mg per day for adult women, according to the NIH ODS - amounts you can get from food and small supplements well below the limit.
What this adds up to
For adults trying to shorten an active cold: start zinc acetate lozenges within 24 hours of the first symptom, dissolve them in the mouth every two hours while awake, aim for a total daily dose of 75 to 92 mg of elemental zinc, and stop when the cold resolves. Expect a metallic or bitter taste and some nausea - these are normal at this dose and not signs of harm that mean you should stop.
Research does not support continuing past the acute illness, using low-dose forms, substituting capsules for lozenges, or starting more than 48 hours after symptoms appear. The dose needs to be high enough. The timing needs to be early. And you need to stop when the cold resolves.
If you take prescription medication - especially fluoroquinolone or tetracycline antibiotics, or penicillamine for rheumatoid arthritis - talk to your doctor before taking high-dose zinc lozenges. These drugs and high-dose zinc can interact and affect absorption. Use the same caution if you are pregnant or breastfeeding.
For more evidence-based writing on immunity and wellness, browse the Ayurnomics collection. More articles from a science perspective are in the Journal.
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