The study found a "significant association" between depression and patients being treated for localised prostate cancer (PCa) with a hormone therapy called androgen deprivation therapy (ADT).
Researchers at Brigham and Women's Hospital (BWH) analysed data from a database of 78,552 men over the age of 65 with stage one to three localised prostate cancer.
They found that patients who received ADT had a 23% increased risk of depression, a 29% increased risk of inpatient psychiatric treatment, and a 7% increased risk of outpatient psychiatric treatment when compared with patients not being treated with ADT.
The study also found that the risk of depression increased with the duration of ADT, from 12% with less than six months of treatment to 26% from seven to 11 months.
Patients being treated with ADT for 12 months or longer were found to have a 37% increased risk of depression.
A similar duration effect was seen for inpatient and outpatient psychiatric treatment.
"We know that patients on hormone therapy often experience decreased sexual function, weight gain and have less energy - many factors that could lead to depression," senior author Paul Nguyen said.
"After taking a deeper look, we discovered a significant association between men being treated with ADT for PCa and depression. This is a completely under-recognised phenomenon. "
According to Nguyen, around 50,000 men worldwide are treated with ADT each year.
"It's important not only for patients to know the potential side effects of the drugs they're taking, but also for the physicians to be aware of this risk in order to recognise signs of depression in these patients and refer them for appropriate care," he said.
"Patients and physicians must weigh the risks and benefits of ADT, and this additional risk of depression may make some men even more hesitant to use this treatment, especially in clinical scenarios where the benefits are less clear, such as intermediate-risk disease."
The researchers have called for future studies to examine whether particular subpopulations are at a higher risk of depression after ADT, such as patients with a history of mental illness.