I didn't want this info. to be buried in other posts, so thought I'd make a new one.
Benadryl is a VERY safe and effective antihistamine that we should all have in our cabinets in case we need it. It is used for allergic reactions and also chronic allergies. It comes in a tablet or liquid. Just make sure that you are using PLAIN diphenhydramine (generic benadryl) and not the combination formulas that have other ingredients.
The standard dosage for dogs is 1mg (milligram) per pound every 8 hours. If you buy the regular tablets, they come in 25mg strength, so obviously that would be hard to divide up for a 5 pound dog. But if that's all you had on hand, you would give a little less than 1/4 of a tablet to a 5 pound dog that was having a reaction.
The children's liquid benadryl is half strength. So it is 12.5mg per teaspoon. So a 5 pound dog would get 5mg, which is a little less than half of a teaspoon. If you have a syringe, it will be marked in milliliters (mm's). 5ml equals ONE teaspoon. So you want to give 2.4ml. (That is approximately half a teaspoon.)
To be completely accurate, try to dose liquids using syringes, as they are much more accurate than the average teaspoon in your silverware drawer.
Or you can measure your teaspoon with a syringe and make sure that 5ml = 1 teaspoon.
I also want to point out that benadryl is very safe and has a pretty wide margin of error. So if you don't dose your dog "exactly" 1mg per pound, they are not going to have any ill effects. Some vets will even go up to 2mg per pound on a dog that is having a bad reaction. So don't stress if you don't get the dosage measured exactly to the tenth of a millimeter.
Hope this helps.