My mom is allergic to benadryl (diphenhydramine) can she us a generic zyrtec?

Maybe, maybe not: These are both antihistamines, just different generation, zyrtec being the younger one. Being allergic to Benadryl (diphenhydramine) is not necessarily meant allergic to zyrtec as well. At times, people misunderstand intolerance vs allergic--people are more often intolerant than allergic. If symptom is mild, she can try zyrtec. If severe, i would consult doc first for eval. Good luck.