Cancer and Supplements: What Vitamins, Herbs, and Botanicals Can (and Can't) Do

5 reasons there aren't a lot of answers—and 4 things you can do until there are.


Few things in medicine get simpler the more you investigate them, and the use of vitamins, minerals, and herbal and botanical supplements to prevent or treat cancer is no exception. Recent clinical trials, for example, suggest that supplements of single nutrients like vitamins B, C, and E and the mineral selenium do not, as once thought, prevent chronic or age-related diseases including prostate and other kinds of cancer. Some substances, like green tea and ginger, seem to have potential in preventing or helping to treat cancer, but they may also actually interfere with treatment or have other serious side effects. Meantime, countless substances that kill or slow the growth of cancer cells in a test tube have not shown that same success in human beings.

What's going on? As it turns out, the question of whether—and in what form—nutrients can be extracted from food or plants and used to fight cancer is quite complex. Researchers in this young field are probing the connections and contradictions but have not yet found answers to the question of what vitamins, herbs, or botanicals may help prevent, treat, or ameliorate symptoms of cancer. Here are a few reasons—plus guidance on what to do in the face of incomplete information.

It's hard to know what to study. You'd think it would be easy to test whether, say, something as straightforward as vitamin E has a role in cancer prevention. But the difficulties begin at once: Which form do you use? Should a study use supplements of alpha-tocopherol, the form most easily absorbed by humans? Or a mixture of the vitamin's eight forms? It gets even more difficult to identify the best component—or components—to study in herbals and botanicals, which contain many different compounds.

And if you want to study supplements together for cancer prevention, what might work best with what? "Unless we have a very clear idea of the pathways and mechanisms that cancer is using, it's hard to have a very strong, rational basis for choosing combinations" of antioxidants or other nutrients, says Peter Gann, director of pathology research at the University of Illinois in Chicago. The problem exists in treatment, too. With so many chemotherapy regimens and so many herbs or botanicals that might theoretically aid their action, what combinations should be studied?

Clinical trials in humans often turn up results different from what happens in the lab. Curcumin (derived from the spice turmeric), quercetin (a kind of plant pigment), and a host of other food-derived chemicals, vitamins, and herbal products have been shown to kill or slow the growth of cancer cells in a lab. Whether these positive results translate to human beings is an entirely different question—as with drugs, many things that are promising in a test tube or in animals fail miserably when studied in the complexity of a human being.

In addition, those human studies are particularly tough to fund and pull off. Supplement makers rarely have an incentive to support research, since it's already legal for their products to be on the shelves. That same availability also means that people assigned to the no-supplement control group may opt to take the supplement under study on their own, contaminating the results. Work is being done, but drawing conclusions will take time. "It's very important that people understand there's research going on but that not all the answers are in," says David Rosenthal, medical director of the Zakim Center for Integrative Therapies at Dana-Farber Cancer Center in Boston.

Your supplements may vary. The pills and capsules you pick up off the shelf of a health-food store can vary enormously in quality and dose. About 25 percent of supplements tested fail when evaluated by whether they contain the claimed amount of a key ingredient, can be readily absorbed by the body, and are free of harmful contaminants, says Tod Cooperman, president of, an independent lab that tests supplements. When his lab tested five green tea products, for example, two failed. (One brand contained as much caffeine as two cans of cola despite saying otherwise, and it had half the active ingredients it claimed.)

Corrected on : An earlier version of this article misstated Peter Gann's title. He is director of the division of pathology research, not director of pathology.