Prostate cancer is one of the few cancers for which patients have a choice of treatments—such as surgery, radiation, hormone therapy, a combination of these, or watchful waiting. In choosing, men of course consider each treatment’s potential side effects—for instance, urinary incontinence and erectile dysfunction.

But there is another side effect that’s not discussed much—shrinkage. Yes, according to a new study, some men notice a reduction in penis size after treatment. And it really bothers them.

Sizing up the research: The study included 948 prostate cancer survivors. Patients and their doctors completed questionnaires about the men’s post-treatment experiences, such as “treatment regret”…sexual function…personal relationships…and other complaints, including any change in penis size.

Results: Nearly 3% of the men said that their penises had become smaller. (Though this study did not specifically gauge how much smaller the men’s penises got, previous research suggested that average decreases in flaccid, stretched length ranged from about one-quarter inch to about one inch, and that penis thickness was reduced as well.)

In this study, the researchers don’t know whether the reported shrinkage was based on flaccid or erect size, and nobody actually measured the men’s penises before and after treatment—so the results are based on the men’s perception of size. But men in general do seem to be quite familiar with this particular part of their anatomy. And perception matters in terms of patient satisfaction. Compared with study participants who did not notice a difference in penis size after treatment, those who did report shrinkage were more than three times as likely to regret their treatment decisions…and more than twice as likely to report that treatment side effects were interfering with their close emotional relationships and overall enjoyment of life.

Which treatment did what: Not all treatments affected the perception of penis size the same way. None of the men treated with radiation therapy alone, whether from an external radiation machine or from implanted radioactive seeds, said that they noticed any size difference. However, shrinkage was noted by 3.7% of the men treated with prostatectomy…and 2.7% of men treated with radiation plus hormone-blocking (androgen deprivation) therapy. Researchers believe that such shortening may be the result of damage to nerves, muscle, vascular structures and/or erectile tissues.

Though it was only a small percentage of study participants who mentioned this complaint to their doctors, it’s possible that there are a lot more men out there who experience this side effect but never discuss it. Still, measured against the consequences of a potentially lethal disease, many men may consider shrinkage to be no big deal.

Bottom line: If you are facing prostate cancer (or if the man you love is) and you are concerned about this possible side effect of treatment, don’t be bashful—discuss this with your doctor and with your intimate partner as you weigh the pros and cons of the various treatment options. Whatever treatment you end up choosing, simply knowing about the possible side effects in advance may make those side effects easier to accept afterward…and increase the odds that you won’t regret your choice down the road.