Hello, I'm Dr. Deb and will do my best to help you today.
I'm not certain if you're located in the UK although I assume that you are. I'd predicate my response on this assumption.
I frequently suggest anti-histamines for allergic cats although I often tell owners that this particular class of drug may not be strong enough to help alleviate symptoms...depending on the nature of the allergy. But, they're always worth a try.
But, to answer your question, Chlorpheniraine (my dose is 2-4 mgtwice a day ) is sold under different brand names such as
Aller-Chlor, Chlo-Amine, Chlor-Trimeton Allergy, Efidac 24.
Two other options for you to consider are:
Claritin (Loratadine) at a dose of 2.5-5 mg/cat or
Zyrtec (Cetirizine) 5 mg once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat). So, you'll want to read the labels closely before you purchase any of these medications.
Sedation is a common side effect with these kinds of drugs.
Benadryl (Diphenhydramine) at a dose of 1/2 of a 25 mg tablet given twice daily is available here in the States but I understand that it's not easy to find in the UK.
Here in the States, these medications can be purchased at most human pharmacies or are even available online although I'm not certain if the same access is available to residents in the UK.
I hope this helps. Deb