Can years of prednisone cause tooth decay?

Yes: Yes there is potential though I have not seen full study on this. Prednisone do have side effect of osteoporosis which is thinning of bones. And so it can theoretically affect the bones of the facial structures and the mouth.

Not a direct cause.: Prednisone or glucocorticoid lower your immune response so you are more susceptible to fungal infection, etc. Tooth decay is caused by bacteria in your mouth. The best defense against tooth decay is removing the bacterial colonies on your teeth and gums by daily regimen of proper brushing and flossing. Also your dentist can place Fluoride varnish after teeth cleaning.

Steriods and teeth: Steroids like Prednisone do not cause tooth decay directly but do have an effect on the gums potentially causing recession and exposure of the tooths root that is not covered by enamel and this area can decay.

Not cause it but..: Years of Prednisone can diminish your natural immunity which can allow bacteria to cause more rapidly advancing tooth decay.