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Homeopathic Nasal Zinc Linked to Loss of Smell 

However, there are no randomized, controlled clinical trials that conclude a loss of smell is one of the possible outcomes from using these products, making it harder to prove cause-and-effect.

Since it would be impossible, as well as unethical, to try to conduct such a study now, Davidson and his colleague, Dr. Wendy Smith, applied the "Bradford Hill Criteria" to 25 patients they had seen for the sudden loss of smell after using a zinc gel product.

The Bradford Hill Criteria were developed in 1965 by a statistician who wanted to establish a causal link between tobacco smoking and lung cancer. The nine key criteria necessary to find a causal link include: strength of the association, consistency, specificity, timing, dose-response (does more of the substance make the problem worse?), biological plausibility, biological coherence, experimental evidence and analogy.

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