PUBLIC HEALTH
July 27, 2010

The Nose Knows

Zinc nasal sprays, often used as cold remedies, appear to damage, and in some cases, destroy, people's sense of smell.

People who use nasal zinc preparations such as zinc gluconate gel to prevent colds are taking a chance on permanent loss of or damage to their sense of smell. This is the conclusion of a study done at the University of California, San Diego.

The study looked at both clinical reports of 25 patients at a nasal dysfunction clinic and at published scientific literature.

Studies of the effect of zinc on cold severity and duration have been inconclusive.

Because zinc has been shown to boost certain immune functions, zinc preparations have become popular as an over-the-counter cold preventative and remedy. Studies of the effect of zinc on cold severity and duration have been inconclusive.

In 2009, the FDA issued a warning to consumers to stop using three intranasal zinc products because of reports that they lead to a long lasting or permanent loss of smell.

Many scientific studies are unable to link cause and effect; they merely suggest that an effect may possibly be due to a certain cause. In the nasal zinc study, the researchers attempted to show a stronger than usual linkage by using the Bradford-Hill criteria: an approach for assessing whether an environmental exposure is the likely cause of an effect or symptom.

One of the criteria is timeliness. Here, the affected patients reported a burning sensation in the nose immediately after zinc usage and a perceived loss of sense of smell within minutes to hours of the burning sensation. This is a positive temporal (time-related) association.

That most patients were otherwise healthy and used no other cold treatment other than the zinc also suggests a linkage between zinc usage and loss of smell. And chemicals other than zinc, such as chlorine and ammonia, are also known to have the ability to cause loss of smell.

The nine Bradford-Hill criteria are: strength of association, temporality, consistency, specificity, dose-response, biological plausibility, biological coherence, experimental evidence, and analogy.

Evaluation of all nine criteria supported the idea that intranasal zinc could have been a cause of the patients' loss of smell. Taken together, they strongly suggest that it was the cause.

The effect of zinc on colds remains uncertain. Some studies and analyses have found benefit, others have not. The National Center for Complementary and Alternative Medicine (NCCAM) mentions one 2009 review which concludes that zinc has shown potential for treating colds, and that additional research is needed to determine optimal doses and treatment strategies. While other studies disagree, this one study is likely to keep many people interested in use of zinc as a cold treatment.

But there's no need to use a nasal zinc preparation; zinc is available in other forms, such as lozenges. None of these other forms has been suggested to cause problems with the sense of smell.

An article on the association between nasal zinc and loss of smell was published in the July 2010 issue of Archives of Otolaryngology-Head & Neck Surgery.

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