Millions of North Americans take zinc lozenges to treat their colds. A study in the Archives of Internal Medicine (november 1997) showed that they neither shorten the cold nor lessen its symptoms. Another study from the University of Virginia showed that zinc nasal sprays do not prevent or treat the common cold (Clinical Infectious Diseases, 2001, Vol 33, Iss 11, pp 1865-1870).
Of six controlled studies, three showed some lessening of symptoms and three found no benefit (Annals of Internal Medicine ,125: 2 (July 15, 1996):81). When the authors combined the results of the six studies, they found no benefit from the zinc lozenges. They report that it is impossible to do double blind studies because zinc lozenges have a very distinct metallic taste, causing many people taking zinc to drop out of the study. Those who remained in the studies noticed the distinctly metallic taste and may have felt that they are given the test drug and were supposed to feel better. On the other side, the three studies showing no benefit were criticized for using low doses or different ineffective forms of zinc.
Zinc inhibits the growth of viruses that cause the common cold in a test tube. However, your body is different from a test tube. Cold viruses have persisted for millions of years because they adapt to live in the mucous membranes of human noses and throats, and therefore are passed from person to person through handshakes, sneezes and material. Your body defends itself by producing antibodies and white blood cells, but only after your immunity has been infected with each virus and it produces cells and proteins specific to kill each cold virus. At this time, nobody really knows if zinc lozenges or nasal sprays may be able to treat the common cold because the virus could respond to higher concentrations, different vehicles or more frequent use.
In 1849, an article in the British Medical Journal recommended treating a cold by nailing a hat on the wall and going to bed and drinking spirits until you see two hats. One hundred and fifty years later, we don't have a much more effective treatment.