Key messages
- There may be little or no reduction in the risk of developing a cold with zinc supplements compared to placebo.
- For people who already have a cold, there may be a reduction in how long the cold lasts with zinc supplements compared to placebo.
- There is probably an increase in the risk of non-serious adverse events when zinc is used for cold treatment.
What is the common cold?
The common cold is primarily a viral infection of the upper respiratory tract. There is no specific combination of symptoms that defines a cold, but symptoms can include nasal congestion, sneezing, sore throat, cough, fatigue, and runny nose, with or without fever. Most people usually recover from this illness without medical treatment; however, there are no definitive treatments to prevent colds or shorten their duration. Given the frequency of colds in adults and children, they are a public health burden and a significant cause of lost work productivity and school absenteeism. Prevention and treatment for the common cold with zinc is an ongoing interest.
What did we want to find out?
We wanted to find out if zinc was better than placebo at reducing the risk of developing a cold and shortening the duration of existing colds. A placebo is a substance similar in appearance to a treatment, but that has no known therapeutic effect. Placebos help assess the effects of the belief that a treatment has been received versus treatment actually being received.
We also wanted to find out if zinc was associated with any adverse effects (e.g. harms), especially harms considered to be potential complications of the common cold. Additionally, we looked at self-reported overall cold severity, individual symptom severity, individual symptom duration, and days missed from work or school.
What did we do?
We searched six databases and two clinical trials registers for studies comparing zinc with placebo for the treatment or prevention of the common cold in adults and children. We compared and summarised the results of the studies, and we found and rated our confidence in the evidence based on specific factors such as risk of bias and study methods.
What did we find?
We found 34 studies of cold prevention or treatment. The studies were conducted in 13 countries, a majority taking place in the USA. Twelve studies were conducted on children (aged under 18 years) and 22 studies were conducted on adults. Most treatments ended with self-reported resolution of symptoms, and the minimum treatment period was five days with a maximum period of 540 days. Most of the studies were funded by an organisation with ties to related commerce/industry or did not report their funding. The remainder were funded by private clinics, non-government foundations, universities, or governmental entities.
We assessed the effects of:
- zinc as a preventative measure; and
- zinc as a treatment measure.
We obtained the following results:
Zinc for prevention
Compared with placebo, taking zinc may make little to no difference in preventing people from catching a cold (9 studies, 1449 people). Preventative zinc also probably makes little to no difference to the length of the cold if one has been caught (3 studies, 740 participants) and may make little to no difference to the severity of the symptoms experienced (2 studies, 101 people). Negative side effects were reported by people taking either zinc or placebo; irregularities in taste and stomach upset were the most common.
Zinc for treatment
Zinc taken for treatment of a cold may reduce the length of time that symptoms are present, by approximately two days, when compared with placebo (8 studies, 972 people). However, we have little confidence in the evidence supporting this conclusion. It is unclear whether zinc makes a difference to the severity of the cold symptoms experienced (2 studies, 261 people). Negative side effects were reported more frequently for those taking zinc as a cold treatment when compared to those taking placebo; irregularities in taste and stomach upset were the most common. Studies administering intranasal zinc did not report any cases of anosmia (loss of sense of smell) but information about specific side effects is uncertain.
What are the limitations of the evidence?
Our confidence in the evidence is mostly low to very low, and the results of further research could differ from the results of this review. Several factors reduced our confidence in the evidence. Firstly, some studies did not report adequately on how people in the studies were randomly placed into treatment groups, meaning that differences between the study groups could be due to differences between the participants and not the treatments. Secondly, some studies used widely different ways of delivering treatments. Finally, due to the large differences between studies in the approaches used to test the effectiveness of zinc, it is likely that additional studies are required before any firm conclusions can be drawn.
How up-to-date is this evidence?
The evidence is current to 22 May 2023.