Researchers have not definitively proven the effect of zinc on colds, but research suggests that zinc prevents the rhinovirus from multiplying in the upper respiratory system, notes Mayo Clinic. As of 2015, research indicates that zinc can reduce the length of a cold by one day.
Research suggests that if zinc is taken within the first 24 hours of experiencing cold symptoms, the duration of the cold can be reduced by one day, according to Mayo Clinic. Zinc attacks the rhinovirus, the virus which causes most common colds, and prevents it from multiplying. When zinc is taken as a lozenge or syrup, it is more effective because it lingers in the throat and nose, preventing the rhinovirus from attaching to the mucous membranes. However, researchers have not been able to identify what the most effective dosing of zinc is to cure colds.
Zinc should not be taken for more than five days, and nasal sprays should be avoided, as they can cause a decrease in a sense of smell, states WebMD. Using zinc for more than six weeks can lead to copper deficiency. Other side effects of zinc usage include a metallic taste or upset stomach.