Can taking antioxidants affect cancer treatment?

by | Breast Cancer, Cancer Care

fruits and antioxidants

Breast cancer is one of the most common cancers in women worldwide.

Faced with a diagnosis of cancer, many women seek out complementary and alternative medicines. The most common alternative medicines taken are dietary supplements which include vitamins, minerals and herbal extracts.

At least when it comes to reducing the risk of cancer, diets high in fruits and vegetables (which are also high in antioxidants) can be protective.

The question of whether high-dose antioxidant supplements can have similar benefits to fruit and vegetables, especially during and after cancer treatment, is a controversial one.

There are concerns that supplements, especially antioxidants, could counteract the effects of chemotherapy or radiation therapy. Radiotherapy and certain types of chemotherapy are designed to increase the production of a type of free radical in the body called reactive oxygen species (ROS).

Women with breast cancer who take antioxidant supplements during their chemotherapy or radiotherapy treatment have been found to have higher rates of mortality and cancer recurrence.

ROS can react with genes inside cells by creating structural bonds which can prove potentially fatal to cells. If the damage occurs to malignant cells then that is bad news for cancer and good news for the person. Unfortunately, ROS action is not always selective, and many normal healthy cells succumb to ‘friendly fire’ from the cancer treatment.

This is where the story gets complicated as theoretically, antioxidant supplementation could either increase or decrease the effectiveness of cancer treatment.

Antioxidants could protect healthy cells from ROS damage, reducing the side-effects of treatment. This would have the benefit of allowing higher doses of treatment for a longer period.

An equally valid opposing view is that high doses of antioxidants could quench many of the ROS species and reduce the treatment’s effectiveness.

In a new research study aiming to answer the question of antioxidant supplements during cancer treatment, researchers collected supplement use habits of over 2,200 post-menopausal women who had undergone breast cancer treatment.

Over a third of the women took supplements before and after their cancer diagnosis. Vitamin C, vitamin E, vitamin A, selenium, zinc and multivitamin supplements were the most common supplements taken.

After adjusting for potential factors that could bias the study, supplement use during chemotherapy or radiotherapy was linked to a 64 per cent greater total mortality several years after the treatment ended. An almost doubling in the rates of cancer recurrence was also seen in supplement users.

The study was observational so cannot prove that it was the antioxidants interfering with the cancer treatment, but the study results do not stand in isolation.

Already recommendations exist advising people undergoing cancer treatment to not take antioxidants and this is based on previous intervention and observational studies with antioxidants during cancer treatment showing worse outcomes.


During breast cancer treatment, the taking of antioxidants is a choice that should be closely discussed with treating medical staff.

While there is a little evidence to show that antioxidants can reduce treatment-related side-effects, this appears to be at the expense of tumour recurrence rates and overall survival.