Need antihistamine dosage help for cat with allergies

My cat has allergies or asthma. She begins wheezing and coughing from bronchial constriction which then gradually becomes an upper respiratory infection. We have had her on steroids in the past and now the vet has prescribed Atopica. Does anyone know how long it takes for Atopica to begin working? It has only been a few days but there doesn't seem to be any improvement and my cat is very lethargic. I would like to try some common human allergy medication like Zyrtec or chlortrimeton but I am unsure on how much to give her. Can anyone help?

Please do not use human medications on your cat. Consult with the prescribing vet about when you can expect the atopica to take effect. Atopica does have some side effects, some that may be short term others that may appear later, with long term use. Discuss your concerns with your vet.

http://us.atopica.com/atopica-for-cats/atopica-for-cats.htm

http://us.atopica.com/pdf/atopica-for-cats-insert.pdf

I would discuss treatment options with your vet.  Is your cat on anything for the asthma?  Here is a site that has excellent information about feline asthma.   http://www.fritzthebrave.com/    My boy Munchie is asthmatic, we have him on steroid inhalers twice a day and a rescue inhaler when he needs it.  We use a device called the Aerokat which helps administering the inhalers easier.   As far as antihistamine doses, check with your vet on that.  I do know that Benadryl doesn't always work in cats, but I've had my other cat on Chlorpheniramine before.