Effectiveness

• The antihistamines routinely prescribed for cats effectively reduce symptoms in about 70 percent of animals and are even more effective when combined with fatty acid supplementation.
  
  

Function

• Diphenhydramine (Benadryl), chlorpeniramine (Chlor-trimeton), and clemastine (Tavist) keep histamines from attaching to a cat's cell receptors so they can't cause allergy symptoms.
  
  

Side Effects

• All three antihistamines may have a sedative effect. Benadryl and Tavist cause dry mouth, while Chlor-trimeton may give your cat diarrhea. Extremely itchy cats might benefit from mild sedation.
  
  

Consideration

• The most effective way to use an antihistamine, advises the ASPCA, is as a preventative. Give it to your cat before he will be in a situation likely to trigger his allergies.
  
  

Warning

• Cats can't tolerate antihistamines containing pseudophedrine, according to the University of Illinois College of Veterinary Medicine.