ADD another disease to the list of ailments that may be thwarted by regular aspirin use — prostate cancer. Researchers reported that men who took at least three aspirin tablets a week reduced their risk of developing or dying from advanced prostate cancer. The aspirin did not affect whether the men developed the disease to begin with, though.

Separately, other scientists found what they described as early evidence that a government panel’s recommendation against routine prostate-cancer screening may be having an unwelcome result: an increase in detection of tumours at more advanced states, when they are harder to treat.

Findings from the two studies were released Monday ahead of their presentation at the American Society of Clinical Oncology (Asco) Genitourinary Cancers Symposium, in San Francisco later this week.

About 220,000 cases of prostate cancer were diagnosed in the US last year, according to American Cancer Society estimates, making it the second-most common cancer among men, after skin cancer. About 27,500 men died of prostate cancer in 2015.

In most cases, the cancer is slow-growing, does not spread beyond the prostate gland and is of little clinical consequence. In such cases, 99% of men are alive after five years and 98% after a decade. Once the tumour spreads to bone or other parts of the body, though, five-year survival falls to 28%.

The aspirin study is a new analysis from the long-running Physicians’ Health Study, a longitudinal trial at Brigham and Women’s Hospital in Boston and the Harvard TH Chan School of Public Health.

Previous prostate-cancer studies have yielded conflicting results on aspirin’s preventative benefits. Researchers analysed data from 22,071 men from the study. During a follow-up of 27 years, 3,193 were diagnosed with prostate cancer, among whom 403 had lethal disease, defined as cancer that had metastasised or spread beyond the prostate or that resulted in death.

The analysis found that regular aspirin resulted in a 24% lower risk of developing lethal cancer after being diagnosed with an early stage of the disease, and a 39% reduced risk of dying from prostate cancer.

But aspirin had little effect when researchers looked at overall incidence of prostate cancer among the participants. "It was after diagnosis of prostate cancer that there appeared to be a benefit," said Christopher Allard, lead author of the study and a urological oncology fellow at Harvard Medical School. "It doesn’t affect the incidence, but it affects the progression."

He termed the findings "compelling," but along with other cancer experts cautioned that observational study did not prove aspirin’s protective role in the disease. Moreover, it is not certain what dose of aspirin was associated with the outcome.

Dr Allard said one hypothesis for the findings is that by inhibiting platelets, aspirin blocks tumour cells from metastasising to the bone. Regular aspirin use is also associated with protection against colorectal cancer and cardiovascular disease, but it comes with gastrointestinal side effects including bleeding that can cause serious complications in some people.

Sumanta Pal, an oncologist and Asco spokesperson, called the results "intriguing" but said patients considering an aspirin strategy to prevent progression of prostate cancer should discuss risks and benefits with their doctors.

The study related to prostate screening looked at thousands of patients at a large urology practice, covering eight counties around New York City, in the US, before and after 2012, when the US Preventive Services Task Force recommended against routine screening with what is known as a prostate-specific antigen (PSA) test. The concern is that most of the cancers the test detects are harmless but lead many men to undergo surgery and other procedures that leave them with life-altering side effects, such as incontinence and impotence.

Among men getting prostate biopsies at the practice, those tests that found cancer rose from 40% in 2010 and 2011, to 45% in 2015, according to the study. The percentage of those cancers considered high-risk — with a Gleason score of 8, 9 or 10 — also increased from 16% of all newly detected cancers in 2010-2011 to 26% in 2014.

Deepak Kapoor, CEO of urology practice Integrated Medical Professionals and a co-author of the study, said the results were "a remarkable shift" in a "very short period of time".

It is not surprising that significant cancers are rising as a percentage of a shrinking total. Reducing the high rate of unnecessary biopsies and finding fewer low-risk cancers that may not need treating were two of the task force’s goals in recommending against regular prostate screening.

But Dr Kapoor said the absolute number of high-risk cancers seen at the practice was rising as well — to 229 in 2015, up from 219 in 2010-2011. "In a single practice where there have been no changes in clinical patterns or interpreting pathologists, this is very disconcerting," Dr Kapoor said.

Still, it was unclear whether the change was completely due to the federal task force’s 2012 recommendations. The study did not record why the men were referred for biopsies, how often they had been tested for PSA or whether the average PSA levels had changed. The study also did not indicate whether there was a change in the stage of cancers found among the men who had prostatectomies.

It will not be clear for many more years if more men will die of prostate cancer. "There are too many potential confounders in this study to be sure of the results," said Peter Carroll, chief of urology at the University of California, San Francisco, who was not involved in the research.

Dr Kapoor said, "I’m not suggesting that we have the definitive answer. But something is going on here that we need to understand better."

The US Preventive Services Task Force is currently reviewing its prostate cancer recommendation for 2017.

More Africa news from The Wall Street Journal

More news from The Wall Street Journal

Premium access to WSJ.com: $1 a week for 12 weeks