Benedryl is an antihistime so it is not going to work as a decongestant. There are not any decongestants over he counter that I know of for dogs. You can try some childrens saline nasal drops and see if that helps. You can also put him in the bathroom and run a hot steamy shower as that may help. If these do not give him any relief a trip to the vet is in order to get him some relief.
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