Studies on human cancer cells show that in cells treated with a form of vitamin C in vitro, chemotherapy drugs killed 30% to 70% fewer tumour cells than usual and researchers say that the same mechanism may affect patient outcomes because vitamin C supplements may undercut the effectiveness of cancer drugs.
Researchers also implanted human cancer cells into mice, and found that when mice were given vitamin C supplements two hours before chemotherapy, the tumours grew more quickly.
Five commonly used chemotherapy drugs doxorubicin, cisplatin, methotrexate, vincristine and Gleevec, also known as imatinib, were tested. Vitamin C did not make the drugs completely ineffective, but it blunted their effects, according to researchers.
The concern is over large amounts of vitamin C taken as supplements, rather than normal amounts of dietary vitamin C.
