Reducing testosterone levels with androgen deprivation therapy, or A.D.T., is a common treatment for prostate cancer. But a new study has found that it more than doubles the risk of dementia.

Previous studies have linked the hormone treatment to an increased risk of depression and Alzheimer’s disease. This new study considered all types of dementia.

Researchers reviewed the medical records of 9,272 men diagnosed with prostate cancer between 1994 and 2013 and who had no previous diagnosis of dementia. They found that the absolute increased risk of dementia after five years was 7.9 percent among those who had been given hormone treatment compared with 3.5 percent among those who had not. Patients who had been receiving A.D.T. for a year or more had the highest increased risk.

The study, in JAMA Oncology, adjusted for smoking, diabetes, cardiovascular disease, cancer and other factors.

This was a retrospective study and therefore cannot prove cause and effect. Still, low testosterone levels have been shown to increase the chance of cardiovascular disease, a known risk factor for dementia.

“Whether or not to use A.D.T. is still a risk-benefit discussion with a physician,” said the lead author, Dr. Kevin T. Nead, a resident at the University of Pennsylvania’s school of medicine. “This study is important and urges us toward future research, but I don’t think it should impact clinical practice.”