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If the itching is caused by an allergy, you can give 12.5 mg of benadryl to get you through the weekend. Just give it every 12 hours. You need to contact your vet on Monday for a better medication.
Let me know if you still have questions.
Antihistamines available over-the-counter are Benedryl and Chlortrimeton. There are prescription antihistamines, notably Atarax (hydroxyzine) that work better in some cases. The benadryl has some drowsiness side effects. Keep in mind that if you choose an over the counter medication that you need to give a pediatric dose.
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The the atarax did not work, there is a good chance that neither of these other medications will work.
You would be best served by returning to your vet for another diagnosis and medications.
If the atarax did not help, my bet is that the cause is not an environmental allergy that you are facing. I would now start to change her diet and be sure that she is on frontline year round.
I do not know the dosage for chlortrimeton but you would give 12.5 mg of benadryl every 12 hours.