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Study probes possible link between intranasal zinc gels and loss of sense of smell
July 23, 2010
By Patricia Nicholson
A new research report suggests a possible link between using over-the-counter nasal zinc gel (zinc gluconate), and complete or partial loss of sense of smell (known as anosmia).
Nasal zinc gel is a homeopathic treatment used to prevent or shorten the duration of the common cold. In background information provided in the research report, the authors noted that there is very little evidence that nasal zinc gel is effective. However, after studying 25 patients who complained of a sudden complete or partial loss of their sense of smell after using nasal zinc gel, the researchers concluded that the treatment may be the cause of the anosmia.
The researchers explored this possible relationship using a set of nine conditions known as the Bradford Hill criteria, which are used to demonstrate causal links between specific factors and diseases. These criteria were established by statistician Austin Bradford Hill, who used them to show the relationship between smoking and lung cancer.
All nine of the criteria supported a causal link between nasal zinc gels and anosmia and hyposmia (reduced sense of smell): the association is strong, consistent, specific, temporal (i.e. the effect always occurred after the suspected cause), displays dose-response (more exposure results in more common or more severe effects), plausible, coherent, analogous (i.e. comparable relationships exist, such as other known substances that can cause anosmia), and is supported by evidence from experiments. - The strength of the association rests not only on the patients’ response to the nasal gel (many reported inhaling it deeply, feeling a burning sensation when they used it, and noticing a loss of sense of smell soon after), but also on known responses to other zinc products used intranasally: the researchers noted intranasal zinc sulphate used in schoolchildren in the 1930s as an anti-polio measure resulted in loss of smell in about 1 per cent of the children, and that stronger zinc sulphate solutions are used to impair sense of smell in animals.
- The association has consistency: it has been observed in several formats in different locations and at different times. Examples include the 1930s attempt to prevent polio, the current study group, and other reports in medical journals.
- The association is specific to loss of sense of smell.
- The association makes sense in terms of timing: the symptom occurs after the use of zinc nasal gel, usually in a specific order (immediate burning, then loss of smell either immediately or over the next several hours).
- The dose response effect – meaning that more exposure to intranasal zinc results in more common or more severe response – has been shown in mice.
- The relationship is plausible, coherent and has analogies. Zinc gluconate is a weak acid that breaks down proteins, so it could degrade the proteins in the olfactory receptor cells (smell receptors). This association coherently fits with existing knowledge about anosmia and does not conflict with any scientific evidence. There are analogous examples of substances – such as ammonia and cadmium – that can induce loss of smell, so such relationships are known to exist.
- Evidence from experiments supports the relationship.
Based on the Bradford Hill criteria, the researchers concluded that it is likely that intranasal zinc can cause reduction or loss of sense of smell. They also noted that it may be wise to apply more stringent regulations to homeopathic treatments that are available over-the-counter or on pharmacy shelves.
The research was published in Archives of Otolaryngology – Head & Neck Surgery.
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