You can take Benadryl, or any antihistamine you happen to have. If you do not have one, you can go to the pharmacy and get one, to take it right away. In a pinch and if you do not have; high blood pressure, are on a salt restricted diet or kidney disease, you can dissolve one level teaspoon of baking soda (not baking powder) into a full glass of water.
This will alkalise your system and should help to stop the reaction. There are instructions on the baking soda box on how to take it internally, but be sure it is completely dissolved before you drink it. Do not use baking soda very often, as it can be hard on the kidneys.
If home treatments do not work within the hour, other parts start to swell, you have trouble breathing or you get worse, call your doctor or go to the ER right away. Good luck.
The quickest way to reduces and completely stop lip swelling that's been caused by an allergy is taking an over-the-counter antihistamine. If you don't want to use medicine you can try applying cold to the swollen area or a thick paste made by mixing backing soda and water. Application of Aloe Vera can also work.