Can Zinc Treat the Common Cold?

Ben Lomaestro, PharmD

Disclosures

November 21, 2011

Question

Is zinc effective for treatment and prevention of the common cold?

Response from Ben Lomaestro, PharmD
Senior Clinical Pharmacy Specialist in Infectious Diseases, Albany Medical Center Hospital, Albany, New York

The common cold is one of the most widespread illnesses, with adults averaging 2-4 episodes per year.[1] Morbidity associated with the common cold is not trivial. The median duration of illness is 7.4 days, with 25% of cases lasting approximately 2 weeks.[1] Although the exact mechanism of zinc treatment for the common cold is unknown, ionized zinc is thought to assist T cells which kill virus-infected cells. Ionized zinc may act by inhibiting the formation of viral capsid proteins and/or by stabilizing and protecting cell membranes.[2,3] The majority of viral replication occurs within 1 day of symptom onset, and this is the time interval for which intervention might be most beneficial, although administration within 3 days may still have benefit.[1,2]

A recent Cochrane review evaluated 13 therapeutic trials (966 patients) and 2 preventive trials (394 patients).[1] Common cold duration was reduced if zinc was taken within the first 24 hours of cold symptom onset (standardized mean difference [SMD], -0.97; 95% confidence interval [CI], -1.56 to -.038, P = .001). Illness severity was also reduced (SMD, -0.39; 95% CI, -0.77 to -0.02, P = .04).

Another recent systematic review of 13 clinical trials evaluating zinc lozenges found similar benefit.[4] The author hypothesized that substantial variation in study outcomes was the result of differences in the 7-fold variation in the total daily zinc dosage. Five of the studies reviewed used < 75 mg/day of zinc and none found a beneficial effect, whereas 7 of 8 comparisons using > 75 mg/day of zinc found statistical benefit. The author estimated that lozenges containing 75 mg/day or more of zinc could reduce the duration of colds by 32% (95% CI, 27%-37%) and possibly more with zinc acetate formulations.

Eby[5] reviewed 15 randomized controlled trials of zinc lozenges and found efficacy to strongly correlate with ionized zinc content at physiologic concentration in a dose-response manner. Best results were achieved with lozenges having only 1 ligand (acetate or gluconate) and consequent high ionic zinc content. A lozenge with the same dose of zinc as another may have a different level of free zinc if it contains substances that bind zinc ions tightly, such as citrate, tartrate, or glycine.[4]Products that do not release ionic zinc may, in fact, make colds worse.[2,5] Zinc ions may also be partially inactivated due to complex formation by components of saliva.[2]

Contact time is also important. Lozenges are thought to be most efficacious when dissolved slowly in the mouth over 20-30 minutes, releasing > 18 mg of zinc and administered every 2 hours.[5] Intranasal administration of zinc is not recommended due to potential loss of the sense of smell (anosmia) which can be permanent.[5] A recent FDA news release advised consumers to stop using intranasal zinc products due to several reports of anosmia.[6]

The Cochrane review also found a favorable prophylactic effect of zinc administration upon the incidence of colds. The incidence rate ratio of developing a cold was less in the zinc group (0.64; 95% CI, 0.47-0.88, P = .006). Zinc supplementation in children resulted in fewer absentee days from school (SMD, -0.37; 95% CI, -0.7 to -0.04, P = .03).[1]

Use of zinc lozenges can result in adverse effects such as nausea (possibly reduced by eating a small amount of food prior to administration), bad taste in the mouth, diarrhea, and dry mouth.[1,2,3,4]The 2011 Cochrane review found zinc lozenges to be more likely to produce adverse effects (especially altered taste and nausea) than syrup formulations but possibly fewer systemic toxicities.[1,2] Other adverse effects are possible. T-cell activity may be impaired by administration of 300 mg of zinc per day for 1 month or more.[3] Large doses of zinc can compete with copper and manganese absorption, resulting in blood abnormalities.[3] In addition, zinc lozenges (and presumably tablets and syrup) may cause drug-drug interactions through chelation of antimicrobials such as tetracyclines and fluoroquinolones.[2]

The Cochrane review concluded that zinc lozenges or syrup administered within 24 hours of symptom onset reduces the duration and severity of the common cold in healthy people. Also, zinc supplementation for at least 5 months reduces cold incidence, school absenteeism, and prescription of antibiotics in children.[1] However, due to differences in study populations, dosages, formulations, and duration of treatment, it is difficult to make firm recommendations about the duration, formulation, and duration of therapy.[1]Use of zinc is advised with caution. Furthermore, due to lack of data, supplementation is not recommended for patients with underlying chronic illness, immunodeficiency, asthma, and other comorbidities for which common cold symptoms might be especially troublesome.[1]

Researchers agree that more investigation is needed to define the optimal dose, duration, and formulation of zinc needed to produce clinical benefit without undue adverse effects.[1,5]In the meantime, zinc acetate lozenges having 18 mg or more of ionized zinc used every 2 hours while awake has been suggested as an effective treatment for the common cold.[5]

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