BACKGROUND: Of the 10 published randomized controlled trials, only 5 reported that zinc lozenges reduced the duration of cold symptoms. The reasons for the different results among these trials include differences in doses, salts, and formulations of zinc lozenges; differences in study design; and difficulty in truly blinding study participants.
POPULATION STUDIED: Fifty ambulatory volunteers older than 18 years were recruited within 24 hours of developing cold symptoms. Potential participants were students, staff, or employees of Wayne State University who had at least 2 of 10 cold symptoms: cough, headache, hoarseness, muscle ache, nasal drainage, nasal congestion, scratchy throat, sore throat, sneezing, and fever. Exclusion criteria included pregnancy, immunodeficiency disorder, any chronic illness, and previous use of zinc lozenges. The treatment groups did not differ significantly in terms of age (37±11 years), however, more patients receiving placebo were smokers (35% vs 20%) and had an allergy history (15% vs 8%).
STUDY DESIGN AND VALIDITY: The appropriate study design was used for this efficacy trial. Two patients in the placebo group dropped out on day 2; all other patients completed the study and were included in the analysis. The patients, the research consultant preparing the randomization code and packages of medications, and a research assistant distributing the study medication were not aware of treatment assignment (ie, allocation was concealed).
OUTCOMES MEASURED: Both patient-oriented (cold symptoms) and disease-oriented (plasma levels of zinc and cytokines) outcomes were used to assess the patients. The primary outcome was average duration of cold symptoms.
RESULTS: Zinc-treated patients had lower overall severity scores (P <.002, test for treatment x time interaction), as well as a significantly shorter mean duration of all cold symptoms, 4.5 versus 8.1 days (3.6-day difference; 95% confidence interval, 2.6-4.6 days; P <.01). By day 4, the average severity score in the zinc group was half that in the placebo group (2.7 vs 5.4 out of a possible 30). Of the 10 measured cold symptoms, only the duration of nasal discharge and cough were significantly shorter in the zinc-treated patients. The average number of lozenges taken per day did not differ between treatment groups (6.2 for zinc and 5.8 for placebo). Patients in the zinc group had a statistically significant higher incidence of dry mouth (P=.003) and constipation (P=.02). Although it did not reach statistical significance, zinc-treated patients experienced more bad taste (52% vs 26%, P=.08).
The zinc acetate lozenge formulation used in this well-designed study was more effective than placebo in decreasing the duration and severity of cold symptoms, especially cough and nasal discharge. However, twice as many zinc-treated subjects correctly identified their lozenges at the end of the study, suggesting that blinding may not have been completely maintained. Also, the 2 groups differed with regard to potential confounders (ie, smoking status and allergy history). Despite these shortcomings, this study adds to the growing evidence that certain zinc lozenge formulations shorten the duration and decrease the severity of cold symptoms.