The standard dosage for oral Benadryl is 1 mg per pound of body weight, given 2-3 times a day. Most drug store diphenhydramine tablets are 25 mg, which is the size used for a 25 pound dog. Always double check the dosage before giving an over the counter medication. In addition, many formulations are combined with other medications such as Tylenol so make sure Benadryl tablets contain only diphenhydramine.