Are Supplements Safe During Cancer Treatment?

NCCIH is the NIH center that studies these products. It puts the position directly. The effects of taking vitamin and mineral supplements during cancer treatment are uncertain. That includes antioxidant supplements. They might interfere with the effectiveness of chemotherapy or other cancer therapies. So anyone in treatment should discuss their use with their health care providers.

"Uncertain" is the honest word. It is not "safe," and it is not "dangerous." It means the studies needed to settle the question have not been done for most products.

The NIH Office of Dietary Supplements adds what oncologists usually do. They generally advise against antioxidant supplements during chemotherapy or radiotherapy. The reason is that these supplements might make those treatments less effective. They may block oxidative damage inside cancer cells. That is the mechanism the concern rests on. Several treatments work partly by causing oxidative damage. An antioxidant is, by definition, something that opposes it.

A systematic review of randomized controlled trials has questioned that concern. The Office of Dietary Supplements still says more research is needed. The open question is the risks and benefits of taking antioxidants alongside conventional cancer therapy. So the caution stands, but as caution rather than proof.

The most useful evidence comes from trials that tested supplements in healthy people, to prevent cancer. They matter for one reason. They show supplements can move outcomes in the wrong direction.

Beta-carotene and lung cancer in smokers. The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study enrolled 29,133 male smokers aged 50 to 69. On average they smoked 20.4 cigarettes a day. They had done so for about 35.9 years. They took 50 mg/day alpha-tocopherol, 20 mg/day beta-carotene, both, or a placebo. Treatment ran 5 to 8 years. Beta-carotene supplements increased lung cancer risk by 18%. Other cancers were barely affected. The overall death rate was 8% higher in the beta-carotene group, mainly from lung cancer and heart disease.

Vitamin E and prostate cancer. SELECT began in 2001. It tested daily supplements of 400 IU of synthetic vitamin E, with or without 200 mcg of selenium. The plan was 7 to 12 years of dosing in 35,533 healthy men aged 50 and older. The goal was fewer new prostate cancers. The trial was stopped in October 2008. Analysis showed the supplements did not prevent prostate cancer, alone or together, over about 5.5 years. Then came the follow-up. It ran another 1.5 years, after participants stopped taking anything. Men who had taken vitamin E had a 17% increased risk of prostate cancer. The comparison was with those who took only placebo, and the difference was statistically significant.

Vitamin E and cancer generally. Two other large trials found nothing at all. HOPE-TOO followed men and women aged 55 and older who had heart disease or diabetes. Over 7 years it compared 400 IU/day of natural vitamin E against placebo. There was no significant difference in new cancers, or in cancer deaths. The Women's Health Study gave healthy women aged 45 and older 600 IU of natural vitamin E every other day, for 10 years. It found no reduction in the risk of any form of cancer.

The lesson is not that supplements are poison. It is that "might help, cannot hurt" is false. Two of the most carefully run supplement trials ever done found harm.

The clearest example is St. John's wort, and NCCIH names the drugs.

St. John's wort can weaken the effects of many medicines. The list names three cancer drugs: irinotecan, imatinib, and docetaxel. It also includes some antidepressants, such as amitriptyline and bupropion. Birth control pills. Cyclosporine. Phenytoin and carbamazepine. Digoxin and ivabradine. Some HIV drugs, including indinavir and nevirapine. Warfarin. And certain statins, including simvastatin.

NCCIH goes further than its usual language for this one herb. It says telling health care providers is especially crucial here. The reason is that this herb interacts with so many medicines. Those interactions can weaken life-saving medicines, or cause dangerous side effects.

More generally, NCCIH notes that herbal supplements may have side effects. Some may interact in harmful ways with drugs, including drugs used in cancer treatment. Some herbs may make chemotherapy less effective.

Vitamin E offers a second, separate hazard that has nothing to do with antioxidant effects.

Vitamin E can block platelet clumping. It also works against vitamin K-dependent clotting factors. Large doses taken with a blood thinner such as warfarin can raise bleeding risk. That risk is higher when vitamin K intake is low. The Office of Dietary Supplements notes the amount needed to matter clinically is unknown. It probably exceeds 400 IU/day.

That matters in cancer care for two reasons. Blood clots are common in cancer, so blood thinners are common too. And chemotherapy lowers platelet counts on its own. One bleeding risk stacks on another.

Many herbal products meant to be swallowed are sold as dietary supplements. NCCIH notes that the regulatory paths for supplements differ from those for drugs. Unlike drugs, dietary supplements are not approved.

That has practical consequences. A supplement bottle carries no drug-interaction section. A prescription label does. What is inside the bottle may not match the label. And the person best placed to check a product against a treatment regimen is the oncology pharmacist. That person is often easier to reach than the oncologist.

One action covers most of the risk. Bring every bottle to the care team before treatment starts, or a written list of everything taken. Do it again whenever something new is added. That includes vitamins, minerals, herbs, and protein powders. It also includes teas taken as medicine, and anything bought online.

Questions worth asking:

• Does anything on this list interact with the specific drugs in my regimen?
• Should antioxidant supplements be stopped during chemotherapy or radiation, and for how long?
• Is there a documented nutritional deficiency here that actually needs supplementing?
• Am I on an anticoagulant, and does that change what is safe?
• Can the oncology pharmacist review this list?
• If I stop a supplement, is there a plan to reassess after treatment ends?

The word "natural" carries no safety information. Beta-carotene is a natural compound found in vegetables. Put in a supplement and given to smokers, it raised lung cancer risk by 18%.

Related reading: Herbal supplements and cancer drug interactions goes deeper on specific pairs. Can supplements cure cancer takes on the stronger claims made for them. And nutrition during treatment covers getting nutrients from food.

• NCCIH — Cancer and Complementary Health Approaches: What You Need To Know
• NIH Office of Dietary Supplements — Vitamin E, health professional fact sheet
• NIH Office of Dietary Supplements — Vitamin A and Carotenoids, health professional fact sheet
• NCCIH — St. John's Wort: Usefulness and Safety
• NCI — Diets, Supplements, and Cancer