Vitamin C for Insect Bites and Allergies: I had a spider bite once that swelled on my forearm like a half-egg under the skin. I took 3-4G of ascorbic acid, the cheap Vitamin C from Sams club, about once every hour and a half or so. By dark the swelling was down to maybe 10%. I continued overnight and all day the next day. During that time, while in the yard working, I felt something on my arm. I looked down to see a "fireant" biting feverishly the back of my hand. I assumed there must be something wrong with him as I didn't feel any burning. Another one bit me later with the same results. I only noticed a feeling like something was crawling on me. The wounds never swelled, turned red or itched! It must have been the massive doses of C circulating in my blood.
I estimated I consumed about 40 grams of C over a 24hr period. Normally that much C will give you severe diarrhea and gas! I had neither.
I also take it for severe allergy flareups with great and quick relief, though only last for 2-4 hours, depending on the intensity of the allergen.