You can give Benadryl to cats. The dose is 1 mg per pound of body weight, every 12 hours, so if he weighs 10 pounds, he would get 10 mg of Benadryl every 12 hours.
You can also try Zyrtec which may be easier since it is once a day. Give 1/4 of a 10mg tablet once a day.
Chlopheniramine 4mg tablets are over the counter. If she is over 5 pounds, give her 1/2 a tablets every 12 hours. If she is under 5 pounds, give her 1/4 a tablet every 12 hours.
She may also have food allergies. Purina cat chow has a lot of corn and fillers in it. You could try a food for cats with sensitive skin. One with Omega acids in it would be great. Or you could get her an omega acid supplement and add it to her food. That helps with allergies and itchy skin.
Below are a few reference links you might find useful:
www.VeterinaryPartner.com/Content.plx?P=A&A=652 food allergies
www.VeterinaryPartner.com/Content.plx?P=A&A=468 food allergy myths
www.VeterinaryPartner.com/Content.plx?P=A&A=2607 itching in cats
She could also have inhalant allergies to things like dust mites or pollen.
The easy treatment is to take her to the vet for a cortisone injection.
If your vet thinks your cat is licking, or pulling his hair out, due to psychological reasons an anti anxiety medication may be prescribed.
www.VeterinaryPartner.com/Content.plx?P=A&A=2565 why is my cat pulling his hair out