There are a few items all dog owners should always have in-stock, and one of them is Benadryl. While I am not a veterinarian, I have found this to be very handy. Of course, with any medication, consult your vet before giving it. While Benadryl is not approved by the FDA for use in dogs and cats, it is commonly accepted by the veterinary community.
The type of Benadryl I use is the regular old normal kind, the original hot pink tablets. Generic works just fine, too. I keep it handy for all sorts of allergic reactions that may crop up from general itchiness to insect bites and bee stings, and you can also use it for travel anxiety and coughs. As far as dosage goes, my veterinarian recommends one 25mg pill for every 25 pounds. This is a higher dosage than a human would take, and my vet has said up to 4-pills can be given to my dogs per day, but make sure to evenly space it with 2 pills in the AM and 2 pills in the PM – my dogs weigh 50 and 58lbs.
Of course, there are things to watch out for. Never give a pregnant or nursing dog Benadryl. Also, do not use Benadryl in animals that are allergic to it or other antihistamines. Do not use in animals with glaucoma, prostate disease, urinary obstruction, and high blood pressure. Do not use in pregnant or nursing animals. Use with caution when given with other CNS (central nervous system)-depressant drugs such as tranquilizers or barbiturates. Always tell your veterinarian and pharmacist what other medications your pet is taking. To learn more about Benadryl and dogs, talk to your veterinarian, also, this 1-800-Pet-Meds page has some good info.