Two relatively large studies of this question, reported in 2003 and 2004, yielded good news for sexually active men: high ejaculation frequency seemed to protect against prostate cancer.
As part of Harvard’s Health Professionals Follow-up Study, 29,342 men between the ages of 46 and 81 reported their average number of ejaculations per month in young adulthood (ages 20–29), in mid-life (ages 40–49), and in the most recent year. Ejaculations included sexual intercourse, nocturnal emissions, and masturbation. Study participants also provided comprehensive health and lifestyle data every two years from 1992 to 2000. The scientists found that men who ejaculated 21 or more times a month enjoyed a 33% lower risk of prostate cancer compared with men who reported four to seven ejaculations a month throughout their lifetimes.
An Australian study of 2,338 men came to a similar conclusion. In all, men who averaged 4.6 to seven ejaculations a week were 36% less likely to be diagnosed with prostate cancer before the age of 70 than men who ejaculated less than 2.3 times a week on average. The study found no connection between prostate cancer and the number of sex partners. (An earlier study, however, found that men who had sex with 30 or more women were two to three times more likely to develop prostate cancer than men with only one partner.)
Further study is needed to investigate possible protective mechanisms. In theory, emptying the prostate of potentially irritating or harmful substances might be one such mechanism. Regardless of the reason why, take comfort in the fact that ejaculation is not only pleasurable, but also may convey health benefits.