I give my 23lb. fella 25mg of benadryl. I suggest you start with half that amount.
When in doubt, always contact your vet! :-)
If your treating allergy symptoms, The Dose is 1mg. per lb. every 8-12hrs. However, if this is a senior dog w/any odd congestion, then it would be best to visit your Vet....For instance, a Heart patient.......Karla