Pickett, Shinn, and Smith (1997) considered evidence for and against ototoxicity from ear drops in a comprehensive review. They noted abundant animal evidence for ototoxicity of aminoglycosides solvents, such as propylene glycol (found in VoSol). They concluded that from animal data, it is clear that most ear drops that are used for treatment or prophylaxis of infection have ototoxic potential and point out the puzzling relative lack of common ototoxicity in humans. They suggest that either ototoxicity is not recognized as hearing loss, is attributed to underlying disease, or because of anatomic differences between animals and humans in the thickness of the round window membrane, there is less susceptibility to topical ototoxicity in humans. They pointed out that there was a lack of well-controlled studies on the use of ear drops in humans.