The analysis included 25 patients treated at the University of California, San Diego Nasal Dysfunction Clinic, which Terrence Davidson directs, who experienced loss of smell after using zinc nasal sprays or swabs to prevent or treat colds. Along with colleague Wendy M. Smith, MD, Davidson applied the nine-point Bradford Hill causation environmental exposure statistical measure to assess the probability that the cold-remedy use caused the loss of sense of smell.
Upper respiratory infections and nasal and sinus disease are major causes of both temporary and permanent loss of smell and diminished sense of smell.
In nearly a dozen cases, however, Courts have found little scientific evidence to support the claim that zinc-containing Zicam nasal products caused loss of smell. Daubert based decisions have resulted in claims not making it to a jury. Davidson himself was rejected as an expert witness when the judge ruled his opinions on specific causation to be “seriously flawed.”
Here's an excerpt from the opinion: