© 2015 Matrix Medical. A number of comorbidities and risk factors complicate the successful management of onychomycosis. Underlying conditions and patient characteristics, such as tinea pedis, age, and obesity, contribute to risk, whereas comorbidities, such as diabetes and psoriasis, can increase susceptibility to the disease. There are limited data on treatment effectiveness in these patients. Here, the authors review post hoc analyses of efinaconazole topical solution, 10%, in mild-to-moderate onychomycosis and present new data in terms of age and obesity. The only post hoc analysis to report significant differences so far is gender, where female patients do much better; however, the reasons are unclear. The authors report significant differences in terms of efficacy in obese patients who do not respond as well as those with normal body mass index (P=0.05) and in patients who have their co-existing tinea pedis treated compared to those in whom co-existing tinea pedis was not treated (P=0.025). Although there is a trend to reduced efficacy in older patients and those with co-existing diabetes, differences were not significant. More research is needed in onychomycosis patients with these important risk factors and comorbidities to fully evaluate the treatment challengse and possible solutions.