The following excerpt is from the Lowchensaustralia website (click for complete article):
Benadryl for Dogs - Chinaroad Lowchens of Australia -
One way to treat bee stings is to give the dog Benadryl (diphenhydramine) by mouth.
Typical dosages: for cats and dogs under 30 pounds, give 10 mg ... dogs 30 to 50 pounds, give 25 mg ... dogs over 50 pounds, give 50 mg. Use only the plain Benadryl formula.
"Any medications need to be discussed with and prescribed by a Veterinarian prior to giving them to your pet to avoid an inadvertent and tragic poisoning."
Benadryl is a very useful emergency drug -- both for humans and for pets. If possible,
speak to your vet about possible emergency insect bite/bug sting scenarios with your vet before they happen -- in case your vet cannot be immediately reached. Minutes count!
Benadryl, while a fairly safe drug, DOES have potential adverse effects and is not for every pet. Benadryl is to be used with extreme caution in patients with glaucoma, prostatic disease, cardiovascular disease, and hyperthyroid, among other conditions. It may also interact with drugs that your pet is taking, so be sure to discuss use of ANY drugs for your pet with your vet first. (Sidenote: some human over-the-counter drugs are toxic or fatal to pets; always discuss with your vet before "self-medicating" for any condition.