A study published in BMJ Open evaluated the use of zinc for the prevention or treatment of acute viral respiratory tract infections (RTI) in adults. As a treatment for colds and flu, zinc reduced symptoms by two days, compared to the placebo. The researchers surmised that of 100 people with upper respiratory infections, 19 of them recovered by day seven as a result of the zinc treatment. Symptom severity appeared to be lower for those treated with zinc—with improvements seen by day three. Those taking zinc experienced an 87 percent lower risk of developing severe symptoms.  

In terms of the prevention of colds and flu-like illness, taking zinc was found to have a modest effect compared to the placebo, with one out of 20 infections prevented. Preventive effects were most significant in reducing severe symptoms. Overall, there was evidence suggesting zinc might prevent RTI symptoms and shorten its duration. Researchers were unable to clearly ascertain the efficacy of different dosages and delivery (oral or nasal spray).