The evidence is mixed as to whether taking flaxseed may play a role in reducing the risk of prostate cancer or limiting its spread. In addition, current data is limited to only a few studies. In one study involving men diagnosed with prostate cancer, 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 preliminary studies report that a low-fat diet supplemented with flaxseed reduced PSA levels in men with enlarged prostate glands (benign prostatic hyperplasia) and in those with prostate cancer. However, those results may have been due to a reduction in dietary fats.
Other studies have shown either no positive effect or a negative effect of flaxseed — and the alpha-linolenic acid contained in flaxseed — on prostate health. So 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.