Zinc works in several possible ways. For example, zinc might prevent the formation of proteins that are needed by a cold virus to reproduce and increase its own numbers. Without duplication of the virus, the cold symptoms will cease. Zinc also may attach to proteins that are located on the edge of a cold virus. This attachment impairs the ability of the virus to enter the body's cells, notably those in the respiratory system (the nose, throat, and lungs). Zinc Gluconate has been shown to attach to specific receptor sites on the bacteria that produce Volatile Sulfur compounds, thereby inhibiting their ability to produce these compounds. These sulfur compounds have been shown to interfere with the body's healing process.
Finally, in an undefined manner, zinc salts may protect and stabilize the lining of the cells, which thereby reduces the chance that the virus will penetrate the cells.
Since 1984, twelve controlled studies (referred to as double-blind, placebo-controlled, randomized clinical trials) were conducted to evaluate the usefulness of zinc in treating the common cold. During a double-blind, randomized trial (which is considered by scientists as the most objective and reliable study method) neither the physician nor the patient knows whether the patient is being treated with zinc or a placebo. Which patients receive the zinc or the placebo is assigned in a random order (like the flip of a coin). The placebo in these studies is an inactive, non-zinc lozenge that is made to look like, and in some instances, taste like the zinc lozenge.
Five of the randomized clinical trials (which were done in adults) determined that zinc was effective in treating the common cold if the lozenges were started within 24 hours of the onset of cold symptoms.
In one study, the lozenges reduced the severity of symptoms (assessed by using a scoring system). In the other four studies, the duration of symptoms was decreased by 3 to 4 days.
In another study, the treatments with zinc or the placebo were begun before the volunteer subjects were purposely injected with a cold virus. The severity of cold symptoms was significantly less in the group taking the zinc than in those taking the placebo.
The only form of zinc which has been proven to be effective is zinc gluconate.