Earlier studies suggested that Vitamin C supplements were beneficial for cancer patients. A study also showed that high doses of vitamin C through IV-injection can slow cancer growth by 50%. 

But a recent US study showed that Vitamin C is not beneficial for cancer patients as it reduces the effect of a range of anti-cancer drugs on cancer cells. This study was done by researchers at Memorial Sloan-Kettering Cancer Center.

Researchers used animal models to test the affect of vitamin C on the effectiveness of a range of anti-cancer drugs. Researchers found that every drug they tested did not work equally well if cells were pretreated with vitamin C as they did on untreated cancer cells. Depending on drug tested, 30% and 70% less cancer cells treated with vitamin C were killed. The amount of vitamin C equivalent in high dose supplements was used in this study.

Follow-up tests showed tumours grew more rapidly in mice that were given cancer pretreated with vitamin C while chemotherapy kept untreated cancer in check. 

Researchers explained that some chemotherapy drugs kill cancer cells with oxygen free radicals they generate and vitamin C, as an antioxidant, neutralizes these chemicals reducing efficacy of the treatment. 

Researchers found that vitamin C appeared to protect tiny structures inside the cancer cells called mitochondria from damage.  

Dr Mark Heaney, the lead researcher 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."  

Various experts believe that more research is needed to prove adverse effect of vitamin C on cancer patients. 

Dr Joanna Owens, of the charity Cancer Research UK, said the study was interesting, but at an early stage. She added that 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.