The blend of eight herbs, used by an estimated 10,000 men and sold over the counter, appears to reduce signs of tumor growth in patients who have exhausted all conventional treatments. The product, called PC SPES, may have both benefits and risks. Most common are breast tenderness and enlargement because of herbs that act like estrogen. But 2% to 4% of patients also run the risk of blood clots, a potentially fatal problem, the studies found.

In his study of 69 men, published this month in the Journal of Urology, 88% of patients taking the product experienced a significant drop in a protein called prostate specific antigen, or PSA, which is a marker for tumor growth.