Depends. The term "allergy rash" is non-specific. If it is poisen ivy or ecxema, a rash characterized by dry, scaly patches where itchiness is a major symptom then antihistamines such as Claritin or Benadryl (diphenhydramine) may be helpful. Zyrtec, in my experience, is actually better for itchiness. In more severe cases, topical or oral corticosteroids may have to be used.