Thanks for writing your query.
Yes prolonged use of benadryl may show its side effects on teeth.These side effects may include=
because of sweetening agent of this cough syrup.
,very common side effect,leads to dryness of oral mucosa
,difficulty in swallowing and speech,dental caries etc......
3.staining of teeth.
So i would suggest you,visit your dentist
and go for thorough scaling and polishing of teeth,if staining present.
One more thing avoid prolonged use of benadryl,use this cough syrup when ever required only,not always.
Hope this will be helpful.
Dr. Gunjan Gupta