Every time I speak about semen retention, someone asks me about ejaculation frequency and prostate cancer. Usually, they're referring to a Daily Mail headline from 2017 which reads, “Ejaculating at least 21 times a month significantly reduces a man’s risk of prostate cancer.”
This article references a 2016 study by Rider et al. which found more frequent ejaculation is associated with a decreased risk of prostate cancer. Interestingly, an earlier study of the same cohort of men found ejaculation frequency is not statistically significantly associated with prostate cancer (Leitzmann et al., 2004).
In 2008, Dimitropoulou et al. found frequent ejaculation in younger life increases risk of prostate cancer, but is protective against prostate cancer when older. Moreover, there have been several recent systematic reviews regarding the connection between ejaculation frequency and prostate cancer, all reporting inconclusive findings.
One review found there to be no universally accepted themes and insufficient agreement about the relationship between ejaculation frequency and prostate cancer (Aboul-Enien, Bernstein, & Ross, 2016). Another review found no linear association between ejaculation frequency and the risk of prostate cancer (Jian et al., 2018).
As one review noted, least convincing evidence is available for the interaction of sexual activity and prostate cancer, and well-conducted and longitudinal studies are clearly necessary to evaluate whether the suggested associations between prostate cancer risk and sexual behavior are real or spurious (Brookman-May et al., 2019).
Depending on which systematic review you want to believe, you can say a number of different things about ejaculation frequency and prostate cancer. My opinion, based on the most recent review conducted in 2019, is that we simply cannot tell if there is a causal relationship.
In any case, there are better ways of caring for your prostate than just changing how many times you ejaculate. Instead of worrying about ejaculation frequency, if you're really concerned you should focus on not smoking or drinking alcohol as well as eating a good diet and doing regular prostate massages.