If benadryl (diphenhydramine) caused palpitations and raised heart rate should i be concerned?

Med Side Effect: Everyone reacts differently to certain medications. The most common side effect of Benadryl (diphenhydramine) is usually sedation but some people find it causes a paradoxical effect. The symptoms you describe do not sound like an allergist reaction. If you require an antihistamine try a non-seading antihistamine like Zyrtec, Allegra, or Claritin and see if you tolerated these better.