http://www.philly.com/mld/inquirer/living/health/14842932.htm This is an absolutely fascinating article. I recommend the whole thing.
The distinction between oral and intravenous is crucial. The body automatically gets rid of extra C through urine. Levine's lab has shown that, at high concentrations, the vitamin is toxic to many types of cancer cells in lab dishes. But to get that much C into the body before it's eliminated, it must be put directly into the blood.
This may explain the defining setback of Pauling's crusade. He and his collaborator, Scottish surgeon Ewan Cameron, gave C intravenously and orally, and claimed many of their cancer patients lived surprisingly long and well. In the 1970s, two rigorous government studies intended to test their claims gave only pills - and found no benefits.
How could so many smart people, including Pauling, ignore a variable as basic as the body's ability to absorb and clear a drug?
"I don't want to impugn anyone," Levine said. "It's one of these things where somebody didn't ask the right questions."