Is it true that some vitamins or supplements can cause cancer?

Answer:

In general, getting sufficient, but not excessive, amounts of vitamins and minerals from your diet and supplements may reduce your risk of cancer, while inadequate or excessive intakes may increase the risk. 

Supplementation with beta-carotene (which your body can convert to vitamin A, as needed) may increase the risk of lung cancer in smokers and people exposed to asbestos. The effects of supplementing with retinol (pre-formed vitamin A) on cancer are not clear, although it has been associated with a reduced risk of melanoma in women.

In men with prostate cancer, use of selenium from supplements was shown to increase the risk of death from prostate cancer in a study, although not the overall risk of death. However, in people with selenium deficiency, which is rare in North America, selenium supplementation may reduce deaths from cancer.  

In people deficient in vitamin E (rare in North America), vitamin E supplementation may reduce the risk of liver cancer. Similarly, low-dose vitamin E may help prevent prostate cancer. However, high-dose vitamin E may increase the risk of prostate cancer, as well as bladder cancer recurrence.

High-dose vitamin B-12 was found to increase the risk of colorectal cancer in older adults.

Adequate intake of folic acid (vitamin B-9) lessens the risk of developing certain cancers, but high intake from supplements or fortified foods may increase the risk of prostate cancer.

Maintaining adequate (but not excessive) blood levels of vitamin D may help to prevent certain types of cancer, although high levels of vitamin D in men with very high calcium intake were shown, in one study, to increase the risk of developing prostate cancer. High intake of calcium itself may increase the risk of prostate cancer, although the evidence is mixed. Calcium supplementation may reduce the risk of colon cancer.  

Taking a modest (i.e., not high-dose) multivitamin may reduce the risk of prostate cancer in men, although not deaths from prostate cancer. Women who took multivitamins and had invasive breast cancer were less likely to die of the disease during a 7-year study than those who did not take a multivitamin.

As for other types of popular supplements, high blood levels of the omega-3 fatty acids DHA plus EPA and DPA were found to be associated with increased risk of developing prostate cancers but this does not appear to be a cause-and-effect relationship as taking fish oil supplements and eating fish have not been found to increase this risk. In fact, higher consumption of fish has been associated with a reduction in deaths from prostate cancer. In addition, use of fish oil supplements may reduce the risk of colon and breast cancer. 

Use of muscle-enhancing supplements has been linked to an increased risk of testicular cancer -- although this may be due to the addition of nonconventional ingredients, such as steroids, in some supplements.  

Curcumin has been shown to reduce the number of pre-cancerous changes that can occur in the colon.

Consuming extra virgin olive oil may reduce the risk of breast and colon cancer.

Consuming chocolate has not been found to reduce the risk of cancer, but it has been linked to an increased risk of colorectal cancer in women — although this may be due to increased adiposity (overweight/obesity), which is linked to colorectal cancer and has been associated with greater chocolate intake.

Preliminary laboratory evidence suggests that nicotinamide riboside, a source of vitamin B-3, might increase the risk and progression of certain aggressive cancers, although this has not been confirmed in people.