How can someone take 20,000-100,000 mg of vitamin C a day?

In the website DoctorYourself.com: http://www.doctoryourself.com/allergies.html
it talks about how a guy took 100,000 mg of vitamin C in a day when he was stung and there were no symptoms, not even swelling. It also talks about a lady taking 20,000 mg of vitamin C a day and her horse and hay allergies were gone. Another person who is allergic to 72 different things takes nearly 40,000 mg a day and their allergies were gone.

How do you get your body to take that much a day? If you do take a megadose how do you know whether it will cause problems for you like calcium oxalate kidney stones and other problems?