Is it actually true that benadryl (diphenhydramine) will help with swelling?

Specific treatment: Benadryl (diphenhydramine) is called an anti-histamine. Histamine is a chemical in certain cells called mast cells and is released when there is an allergic reaction such as hay fever. Histamine causes some inflammation which is why our noses get red, congested due to swelling of the mucus membranes. If the swelling is due to something else like trauma, heart failure, vein disease, then it is of no benefit.