Mega-doses of vitamin C, sometimes dismissed as an "alternative" therapy for people with cancer, may be a plausible treatment after all, a study published this week says.

Researchers found three documented cases of patients with advanced cancers having unusually long survival times after receiving high doses of vitamin C intravenously.

The three cases were confirmed by pathologists at the National Cancer Institute who didn't know that the vitamin had been used as a treatment, says the study, published in the Canadian Medical Association Journal.

These cases are not proof that the vitamin can be used as a treatment for cancer, but the reports suggest high doses of intravenous vitamin C should be reassessed as a cancer treatment, said the researchers, led by Sebastian Padayatty of the National Institutes of Health in Bethesda, Md.

Later in his life, chemist Linus Pauling advocated massive doses of vitamin C for preventing colds and treating cancer. But trials conducted at Minnesota's Mayo Clinic on oral vitamin C and cancer failed to show any benefit.

Mainstream scientists have mostly discarded the vitamin as a potential cancer treatment. However, recent lab experiments have found that high concentrations are toxic to some cancer cells, but not to healthy cells.

Padayatty and the other researchers said the maximum oral dose of vitamin C, 18 grams per day, results in concentrations in the body far lower that the ones toxic to cancer cells. Ingesting more than 18 grams a day results in less C being absorbed into the body.

However, when large doses, 50 to 100 grams, of the vitamin are given directly into the blood, its concentration in the body is 14 times higher than the toxic concentration found in the lab.