Is frequent benadryl (diphenhydramine) usage dangerous?

No: There is a common misconception that frequent use of antihistamines leads to tolerance and eventual lack of effectiveness. This is untrue. Frequent use of decongestants that can lead to tachyphylaxis (lack of effect after several consecututive days of use) but there is no real justification for changing back and forth from Allegra to Claritin (loratadine) or to zyrtec. Typically if one works poorly, all will.

Depends: Any medication used in excess or not following the instruction recommendations can be dangerous. Use according to directions should be safe.