Why must you rinse your mouth after using advair disc inhaler?

Steroids can cause overgrowth of a fungal Candida infection commonly called thrush. Normally a person always has both good and bad bacteria in their mouth. Usually there is a balance between them which prevents overgrowth but the steroid residue feeds the fungus and causes overgrowth so rinsing the mouth helps to remove the drug residue that encourages this growth.

Because you don't want to leave the steroid in your mouth, it will cause thrush.