Dosing is done by milligrams. The easiest method is to offer the tablet form of Benadryl. And it is easy to remember, because the dosing is 1 mg for each 1 pound of body weight: Here is an easy chart to follow: Chihuahua Weight Dosing Amount .5 lbs. (.22 kg) .5 mg 1 lb....
Swelling like this is often caused by some sort of allergic reaction - usually to something like a bee or wasp sting. In mild cases the benadryl really should work. However, she might need a higher dose. Benadryl is usually given at 1mg per pound so if she is 50lbs your dose of 50mg...