Chlorpheniramine is a first generation antihistamine used in cats for the treatment of histamine-mediated itchy (pruritic) and allergic conditions. Drowsiness and/or sleepiness can occur with this medication, but this effect will usually lessen with time. Rarely, some cats may become excited after receiving this medication. Gastrointestinal side effects like constipation, diarrhea, vomiting and decreased appetite are rare but have been reported. Chlorpheniramine is given as an oral tablet once to twice daily and can be given with or without a meal. Antihistamines should be used on a regular ongoing basis. If any unwanted side effects or changes in behavior or health are detected, this medication should be discontinued and our veterinarians should be notified.