McGwin et al¹ suggest that treatment of erectile dysfunction with sildenafil (Viagra) or tadalafil (Cialis), two phosphodiesterase inhibitors (PDE5-I), may increase the odds of non-arteritic anterior ischaemic optic neuropathy (NAION) in men with a history of myocardial infarction (MI) or hypertension (HTN). We believe, however, that this study is fraught with significant limitations (as the authors acknowledge) that preclude drawing any conclusions about this relation.

Case-control studies are susceptible to several biases that must be carefully considered and controlled for in the study design, implementation, and analysis.² For example, accurate and complete ascertainment of exposure is critical in a …