There is limited evidence to support the idea that taking flaxseed might lower your risk of prostate cancer.
Some studies have suggested that flaxseed may have positive effects on the prostate. For instance, one study involving men diagnosed with prostate cancer found that those who took a flaxseed supplement and followed a low-fat diet saw their prostate-specific antigen (PSA) levels decline. PSA can be a measure of how fast prostate cancer is spreading.
Other studies have shown no positive effect of flaxseed — and the alpha-linolenic acid contained in flaxseed — on prostate health.
Overall, it remains unclear whether flaxseed or flaxseed oil has a role in the prevention or treatment of prostate cancer. Controlled clinical trials may help determine whether or not it's useful in the long term.