The most common form of Benadryl is probably the 25mg tablet. (So, just to give you rough guidelines, small dogs would receive 1/4 of a tablet 2-3 times a day. Medium dogs are started on a Benadryl dosage for dogs at 1/2-3/4 of the 25mg tablet. And so onâ¦).As is true of many medications, the animalsâ individual body chemistry and genetics will determine its response to a particular drug.edium dogs are started on a Benadryl dosage for dogs at 1/2-3/4 of the 25mg tablet. And so onâ¦). As is true of many medications, the animalsâ individual body chemistry and genetics will determine its response to a particular drug.