Vitamin C supplements may substantially reduce the benefit from a wide range of anti-cancer drugs, research suggests.
Thirty to 70% less cancer cells in a lab were killed by a range of drugs, after pretreatment with vitamin C.
Follow-up chemotherapy tests found tumours grew more rapidly in mice given cancer pretreated with vitamin C.
Cancer Research reports the Memorial Sloan-Kettering Cancer Center findings. The US researchers say the same mechanism may affect patient outcomes.
Some studies have suggested that because vitamin C is an antioxidant it could be beneficial to cancer patients.
The US team tested the impact of a form of vitamin C on the effectiveness of a range of anti-cancer drugs in tests on cancer cells in the lab.
They found that every drug they tested did not work as well if cells were pretreated with vitamin C as they did on untreated cancer cells.
Between 30% and 70% less cancer cells treated with vitamin C were killed, depending on the drug tested.
Follow-up tests on mice showed that while chemotherapy kept untreated cancer in check, tumours grew more rapidly in mice that were given cancer pretreated with vitamin C.
Some classes of cancer drugs produce molecules known as oxygen free radicals which can react with other molecules in the cancer cell, forcing its death.
In theory, vitamin C could mop up the free radicals, keeping the cancer cell alive despite chemotherapy treatment.
However, the researchers found the key was not that the nutrient was neutralising free radicals.
Instead, vitamin C appeared to protect tiny structures inside the cancer cells called mitochondria from damage.
Mitochondria effectively form the energy-creating boiler room of a cell, and if damaged can lead to its death.
Lead researcher Dr Mark Heaney said: "Vitamin C appears to protect the mitochondria from extensive damage, thus saving the cell.
"And whether directly or not, all anti-cancer drugs work to disrupt the mitochondria to push cell death."
Previous research by the same team has shown that vitamin C seems to accumulate within cancer cells more than in normal cells.
The amount of vitamin C used in the current study were equivalent to that found in high dose supplements.
Dr Heaney said vitamin C was probably good for cells in normal tissue - but its protective effect was completely counter-productive in relation to cancer cells.
He added: "The use of vitamin C supplements could have the potential to reduce the ability of patients to respond to therapy."
Dr Joanna Owens, of the charity Cancer Research UK, said the study was interesting, but at an early stage.
"As yet, there is not enough evidence to know whether antioxidants such as vitamin C are helpful or harmful during cancer treatment.
"It is possible that high doses of antioxidants can make treatment less effective, but until we know for sure our advice is to try and get the vitamins you need through a balanced and varied diet rather than through vitamin supplements."
Pamela Mason, scientific advisor to the Health Supplements Information Service, said no conclusions could be drawn until research was carried out on humans.
She stressed that cancer patients should seek expert advice before taking any product not prescribed by their doctor.