Diflucan is less effective in women with diabetes for treating vaginal yeast infections.
Most vaginal yeast infections are caused by C. albicans which Diflucan is 70-90% successful at treating.
But women with diabetes are more susceptible to infections with C. glabrata and C. krusei...strains that are resistant to fluconazole and other azoles.
Vaginal defense mechanisms might be depressed in women who are immunocompromised due to diabetes, HIV, cancer, etc. This may allow the azole-resistant strains to grow.