Several common supplements can increase the risk of prostate cancer progression, interfere with treatment, or worsen outcomes. The most important ones to avoid or limit include high-dose vitamin E, high-dose zinc, folic acid, and herbal supplements like St. John’s wort, especially during active treatment. Here’s what the evidence shows for each.
Vitamin E at Supplemental Doses
Vitamin E is one of the most studied supplements in relation to prostate cancer, and the results are not reassuring. The SELECT trial, a large randomized study of over 35,000 men, found that taking 400 IU of vitamin E daily increased prostate cancer risk by 17% compared to placebo. That translated to 1.6 extra cases per 1,000 men per year. While that may sound small, it was a statistically significant increase in a supplement many men take specifically hoping to prevent cancer.
The concern extends beyond prevention. High-dose vitamin E acts as a potent antioxidant, and during radiation therapy or chemotherapy, that property can work against you. Both treatments rely partly on generating free radicals to destroy cancer cells. Flooding your system with antioxidants may blunt that effect. Many oncologists advise patients to stop antioxidant supplements during active treatment for this reason.
Zinc Above 75 mg Per Day
Zinc plays a legitimate role in prostate health, and the prostate naturally contains high concentrations of it. But supplementing at high doses tells a different story. A 30-year follow-up study of health professionals found that men taking more than 75 mg of supplemental zinc per day had a 76% higher risk of lethal prostate cancer and an 80% higher risk of aggressive prostate cancer compared to men who never used zinc supplements.
Duration mattered too. Men who took supplemental zinc for 15 years or more had roughly double the risk of lethal prostate cancer, even at moderate doses. The researchers noted that doses up to 25 mg per day appeared relatively safe, but risk climbed with higher intake, particularly at extreme levels. For context, most standalone zinc supplements sold over the counter contain 30 to 50 mg per tablet, and many men stack that on top of zinc from a daily multivitamin.
Folic Acid Supplements
Folic acid is the synthetic form of folate, a B vitamin added to fortified foods and found in most multivitamins. Lab research on prostate cancer cell lines (PC-3, LNCaP, and DU145) has shown that higher concentrations of folic acid increase both the growth rate and the invasive capacity of prostate cancer cells. The cells didn’t just multiply faster. They also became more aggressive, penetrating tissue barriers more readily.
This doesn’t mean dietary folate from vegetables is dangerous. The concern is specifically about supplemental folic acid at doses above what you’d get from food, particularly if you already have prostate cancer. Men who are taking B-complex vitamins or multivitamins containing folic acid should discuss this with their oncologist.
St. John’s Wort and Herbal Supplements
St. John’s wort is widely used for mild depression, but it’s one of the most disruptive supplements when it comes to cancer treatment. It speeds up liver enzymes responsible for breaking down many drugs, including chemotherapy agents like docetaxel and paclitaxel, both of which are used in advanced prostate cancer. In one study, St. John’s wort reduced blood levels of an active chemotherapy compound by 42%, a drop large enough to potentially render the treatment ineffective.
This isn’t limited to chemotherapy. St. John’s wort can also interfere with targeted therapies and other medications metabolized through the same liver pathway. The interaction is so well established that oncologists consider it one of the most important supplements to stop before and during cancer treatment.
Other herbal supplements carry similar risks, though less dramatically. Many herbs have unpredictable effects on drug metabolism, and their exact composition can vary between brands. If you’re on any prostate cancer therapy, whether hormone treatment, chemotherapy, or newer targeted drugs, herbal products deserve extra scrutiny.
High-Dose Calcium
Calcium intake above 1,500 mg per day has been linked to increased prostate cancer risk. Below that threshold, the association largely disappears. This is worth paying attention to because men on hormone therapy for prostate cancer often lose bone density and are sometimes advised to increase calcium intake to protect their bones. The recommended approach is to meet standard calcium needs (around 700 to 750 mg per day from food and supplements combined) without overshooting into the range where prostate cancer risk rises. If you’re supplementing calcium for bone health during treatment, keeping your total daily intake under 1,500 mg is a reasonable target.
Antioxidant Supplements During Treatment
Beyond vitamin E, other high-dose antioxidants raise concerns during active cancer treatment. Vitamin C supplements, selenium, beta-carotene, and similar products all neutralize free radicals, which is precisely what radiation therapy and certain chemotherapy drugs produce to kill tumor cells. The worry is straightforward: if you’re mopping up the free radicals your treatment generates, you may be protecting the cancer along with your healthy cells.
Selenium deserves a specific mention. The same SELECT trial that flagged vitamin E found a non-significant trend toward increased prostate cancer with selenium supplementation alone (a 9% increase that didn’t reach statistical significance). The combination of selenium and vitamin E together showed a smaller increase than either alone, which was unexpected and difficult to interpret. The bottom line is that selenium supplements haven’t shown the protective benefit they were once hoped to provide, and there’s enough signal of possible harm to make them worth avoiding.
Multivitamins at High Frequency
Standard daily multivitamin use hasn’t been consistently linked to worse prostate cancer outcomes. But heavy use is a different matter. Data from the NIH-AARP Diet and Health Study found that men taking multivitamins seven or more times per week had a 32% higher risk of advanced prostate cancer and nearly double the risk of fatal prostate cancer compared to men who never used them. A separate analysis from the Health Professionals Follow-up Study did not replicate this finding at similar doses, so the evidence is mixed rather than definitive.
The practical concern with multivitamins is that they often contain several of the individual supplements already flagged here: folic acid, vitamin E, zinc, selenium, and calcium. Even if the dose of each ingredient is modest, the combination adds up, especially if you’re also taking standalone supplements of any of those nutrients.
What to Stop Before Surgery
If you’re scheduled for a prostatectomy or any surgical procedure, the standard recommendation is to stop all supplements at least 10 days before surgery. This includes multivitamins, herbal products, green tea extract, and any over-the-counter nutritional supplements. Many of these can thin the blood, impair clotting, interact with anesthesia, or affect healing. Your surgical team will typically provide a specific list, but the safest default is to stop everything unless your surgeon explicitly tells you to continue it.
Fish Oil: A More Nuanced Picture
Omega-3 fatty acid supplements have generated conflicting headlines. Some earlier observational studies suggested that men with higher blood levels of omega-3 fats had a slightly elevated prostate cancer risk. But a 2024 randomized trial painted a different picture. Men with low-grade prostate cancer on active surveillance who followed a high omega-3, low omega-6 diet with fish oil supplementation for one year saw a 15% decrease in a key marker of cancer cell proliferation. The control group’s marker increased by 24% over the same period.
This doesn’t mean fish oil is clearly beneficial, as the study was small (100 men) and didn’t show differences in tumor grade or PSA levels. Four patients also withdrew due to side effects from the fish oil. But unlike the supplements listed above, omega-3s don’t carry a clear signal of harm. If you’re taking fish oil, it’s reasonable to discuss it with your oncologist rather than assume you need to stop.