While there is conflicting evidence as to the benefits of oral zinc in reducing the duration of colds, taken in excess zinc can lead to "copper deficiency, anemia and damage to the nervous system" (Mayo Clinic). Zinc can also cause a temporary loss of the sense of taste.

Zinc can further interfere with the action of some prescription drugs, including some antibiotics and the blood thinner coumadin.

A meta-analysis suggests that zinc (most effectively as a lozenge) may reduce the length of a cold by an average of one day (Hemilia H, et al. The effectiveness of high dose zinc acetate lozenges on various common cold symptoms: A meta-analysis. BMC Family Practice. 2015;16:24; Sexton DJ, et al.; The common cold in adults: Treatment and prevention. http://www.uptodate.com/home. UpToDate reviews require subscription.)

As a nasal spray zinc has been linked to a permanent loss of smell in some people.

The Tolerable Upper Intake Level of zinc for adults is generally set at 40 mg/day (less for children and teens). This could be exceeded if one followed the Cold-Eeze package directions.

Note that I am not a medical professional and this is meant as general information, not medical advice.