A 2003 study in the journal Phytotherapy Research by Ramsewak et al. reported on the effects of seven ingredients in a generic medicated chest rub on the fungal pathogens responsible for toe nail fungus. This study was performed at the Department of Horticulture and National Food Safety and Toxicology Center at Michigan State University.
Researchers did not use Vicks VapoRub specifically; they studied the ingredients found in a generic version of the product called "Meijer medicated chest rub." According to Ramsewak et al., the active ingredients studied included camphor, menthol and eucalyptus oil. The inactive ingredients were thymol, oil of turpentine, oil of nutmeg and oil of cedar leaf. Researchers looked at the test tube effects of each ingredient on the dermatophytes that cause onychomycosis, more commonly known as toe nail fungus.
In 2009, Anahad O'Connor reported in The New York Times about the study in an article regarding the use of thymol oil, the extract from the herb thyme, to treat toe nail fungus. O'Connor noted that while thymol was shown to be effective against the fungal pathogens in an experimental setting, no studies of the treatment in humans had been conducted to confirm the efficacy of the product.