In a paper by Stanford University Department of Pediatrics and Medicine, with the University of Virginia School of Medicine, and published in the Journal of Clinical Infectious Diseases, 105 published reports of zinc efficacy in warding off the common cold were studied.
Of the 105 articles reviewed, fourteen were randomized placebo-controlled studies—in other words, proper scientific studies. Of these fourteen studies, eleven featured the qualities of inquiry representative of a proper clinical trial.
In reviewing these eleven trials, the author’s concluded "this structured review suggests that the therapeutic effectiveness of zinc lozenges has yet to be established. One well-designed study did report a positive effect of zinc nasal gel."