From my vet: you can safely give 1 to 2 mg Benedryl per pound of body weight. I have a 50# dog, so that would be two to four tablets, as each tablet is 25mg. If you could find a smaller container of LIQUID Benedryl, it is likely to be absorbed faster than the pill will.
I keep a 10 ml/cc syringe taped to a bottle of the liquid. I've written on the bottle with a felt-tip marker: "For bee sting, give Shilo 20 cc by mouth". (Shilo likes to snap at bees.)
1 tsp = 5 ml/cc = 12.5mg Benedryl, one dose for a 5-10# dog
2 tsp = 10 ml/cc = 25mg Benedryl, one dose for a 12-25# dog
1 Tbsp = 15 ml/cc = 37.5mg Bendryl, one dose for a 18-35# dog
4 tsp = 20 ml/cc = 50mg Benedryl, one dose for a 25-50# dog
My vet said it's hard to overdose dogs with Benedryl, so measurement isn't all that critical. Benedryl isn't all that effective on cats, though.
Sue