Zinc

What it is

Zinc is an essential trace mineral required for catalytic activity in over 300 enzymes and structural stability in thousands of proteins throughout the human body [1]. It plays central roles in cellular metabolism, protein synthesis, DNA creation, and immune system signaling [2] [1]. Because the body has no specialized zinc storage system, regular daily dietary intake is necessary [1]. Primary dietary sources include oysters, beef, poultry, pork, legumes, nuts, and fortified cereals [1]. In dietary supplements and medications, zinc is found in several salt forms, including zinc gluconate, zinc acetate, zinc sulfate, zinc picolinate, and zinc citrate [3] [1].

How it might work

Researchers have identified multiple distinct mechanisms for zinc in respiratory infections and general immune defense [2] [3]. When dissolved as oral lozenges, positively charged zinc ions (Zn²⁺) are released directly into the saliva and oral mucosa [3]. In laboratory models, free zinc ions competitively inhibit rhinovirus surface cleavage and viral attachment to the intercellular adhesion molecule-1 (ICAM-1) receptor on nasal and throat epithelial cells [3] [2]. Zinc also inhibits viral RNA replication within infected host cells [2]. Systemically, zinc acts as an intracellular signaling molecule that regulates T-lymphocyte differentiation, natural killer cell activity, and pro-inflammatory cytokine expression [2].

What the research says

Clinical research on zinc focuses primarily on two areas: treating acute upper respiratory infections and correcting deficiency to prevent systemic illness [4] [5].

For the common cold, randomised controlled trials demonstrate that oral zinc lozenges shorten the duration of acute symptoms if initiated promptly [3] [4]. A meta-analysis of seven randomised trials (575 participants) found that zinc lozenges delivering over 75 mg daily reduced the average cold duration by 33% [3]. Trials using zinc acetate lozenges observed a 40% reduction, while zinc gluconate trials observed a 28% reduction [3]. An individual patient data meta-analysis of three trials (199 participants) confirmed that zinc acetate lozenges shortened cold duration by 2.73 days compared with placebo, without differences across age, gender, or allergy status [6]. High-dose lozenges also shortened nasal congestion by 37% and cough by 46% [7]. The 2024 Cochrane systematic review of 34 trials found that therapeutic zinc reduced cold duration by an average of 2.37 days, though trial heterogeneity was substantial [4]. However, Cochrane analyses revealed that prophylactic daily zinc supplementation had little to no effect on preventing colds, with an incidence risk ratio of 0.93 [4].

For systemic immune health, zinc supplementation has strong evidence for reducing morbidity in populations with high rates of zinc deficiency [5]. A Cochrane systematic review of 80 randomised trials involving 205,401 children found that preventive zinc supplementation significantly reduced all-cause diarrhea incidence by 13% (risk ratio 0.87) and improved overall zinc nutritional status [5]. In therapeutic trials for acute childhood diarrhea, oral zinc shortened illness duration by roughly 10 hours in children older than six months [8].

Macular degeneration. Zinc is one component of the AREDS eye formula. In a 2023 Cochrane review, people with AMD taking zinc were less likely to progress to late AMD over six years (OR 0.83, 95% CI 0.70 to 0.98; moderate-certainty evidence), and the full AREDS formula also slowed progression [9]. In AREDS2's 10-year follow-up, lowering the zinc dose from 80 mg to 25 mg made no difference to AMD progression [10].

Doses used in studies

In trials evaluating common cold treatment, effective regimens administered 80 to 92 mg of elemental zinc per day, typically divided into individual lozenges of 10 to 15 mg dissolved slowly every two to three hours while awake [3]. Trials administering very high doses between 192 and 207 mg per day found no additional benefit over 80 to 92 mg daily (35% versus 33% cold reduction) [3]. Treatment in these trials lasted between 5 and 14 days [6] [4].

For general nutritional maintenance and deficiency prevention, daily recommended dietary allowances are 11 mg per day for adult men and 8 mg per day for adult women (11 to 12 mg during pregnancy and lactation) [1]. In pediatric preventive trials in low- and middle-income regions, daily doses ranged from 5 to 20 mg elemental zinc per day for durations of 2 to 12 months [5].

Eye formula: AREDS used 80 mg of zinc a day (with copper); AREDS2 tested 25 mg against 80 mg and found no difference in AMD progression [11] [10].

Safety and interactions

Zinc has several well-established safety thresholds, side effects, and potential drug interactions [1].

• Common side effects: Oral zinc lozenges frequently cause nausea, mouth irritation, stomach discomfort, and an unpleasant metallic taste [4]. In the 2024 Cochrane review, therapeutic zinc lozenge use was associated with a 34% increase in non-serious adverse events [4]. Taking zinc supplements on an empty stomach often triggers acute nausea and vomiting [5] [1].
• Upper intake limits and toxicity: The tolerable upper intake level (UL) for elemental zinc from all sources in adults is 40 mg per day according to the Food and Drug Administration and the Institute of Medicine, and 25 mg per day according to the European Food Safety Authority [1]. Chronic intake above the UL impairs copper absorption by upregulating intestinal metallothionein, which traps copper in mucosal cells [1] [12]. This secondary copper deficiency can lead to severe sideroblastic anemia, neutropenia, and irreversible neurological myelopathy [12] [1]. Short-term use of lozenges at 80 mg daily for up to one week during an acute cold has not been shown to cause hematological toxicity, but high-dose intake must be discontinued once the cold resolves [3] [1].
• Intranasal warning: Topical intranasal zinc sprays, gels, and swabs can cause permanent loss of smell (anosmia) or reduced smell (hyposmia) due to direct toxic destruction of the olfactory neuroepithelium [13]. The Food and Drug Administration issued a public health advisory warning consumers against intranasal zinc cold remedies [13] [1]. Only oral lozenges or oral dietary supplements should be used [13].
• Interactions with medicines: Zinc chelates with certain prescription medications in the gastrointestinal tract, substantially reducing their absorption [1]. It binds quinolone antibiotics (such as ciprofloxacin and levofloxacin) and tetracyclines (such as doxycycline), which can cause antibiotic failure [1]. Doses of zinc and these antibiotics should be separated by at least two to four hours [1]. Zinc also reduces the absorption of penicillamine, while thiazide diuretics increase urinary zinc excretion [1].
• Pregnancy and breastfeeding: The recommended dietary allowance during pregnancy is 11 mg daily and during breastfeeding is 12 mg daily [1]. Zinc intake should not exceed the adult tolerable upper limit of 40 mg per day in pregnant or lactating women without direct medical supervision [1].
• Underlying health conditions: Individuals with chronic kidney disease have altered trace element clearance and should consult a nephrologist before taking zinc supplements [1]. Individuals with Wilson's disease or hemochromatosis require specialist management, as zinc therapy alters copper and iron metabolism [1].

Eye formula doses. In the AMD trials, AREDS linked zinc to more genitourinary problems in men, while AREDS2 found no difference between the 80 mg and 25 mg doses [9]. AREDS-style eye formulas contain 80 mg of zinc a day — twice the 40 mg upper limit — and combining one with a multivitamin can push zinc and vitamin A above their tolerable upper intake levels [14].

Caveats and open questions

A major limitation of the common cold literature is the substantial variability between trials regarding lozenge composition, blinding adequacy, and symptom measurement [4]. Many commercial lozenges contain flavoring agents such as citric acid or mannitol that bind zinc ions in solution, preventing their therapeutic release in the pharynx [3]. In addition, because zinc has a distinct metallic taste, achieving true participant blinding is difficult, creating potential performance bias in subjective symptom reporting [4]. Finally, while zinc reduces acute cold duration in adults, trials in children have shown inconsistent results for upper respiratory infections [4] [5].