Prostate cancer is a major killer
One in eight men screened for prostate cancer will test positive when they do not have the disease, a major European trial has shown.
A positive result can mean undergoing invasive tests such as biopsy as well as potentially unnecessary treatment.
Screening with prostate specific antigen (PSA) is not routinely offered in the UK but government experts are reviewing evidence from the study.
Cancer Research UK said men should talk about the pros and cons with their GP.
Early data from the European Randomised Study of Screening for Prostate Cancer, which is being conducted in seven countries, showed in March 2009 that deaths could be cut by 20%.
But other recent evidence has cast doubt on the long-term benefits of screening, suggesting some men may end up being "over-treated" for slow-growing disease that would never cause a problem in their lifetime.
Now data from the Finnish part of the European trial has shown that for every eight men screened - tests are being done on a four-yearly basis - one ended up with a false positive result, even with a fairly high PSA threshold.
Those men who tested positive but were later found not to have cancer were twice as likely not to agree to screening in the future even though they were at risk of developing the disease later, the British Journal of Cancer reported.
The researchers have said more research is needed to make screening more accurate and to help pick out those who are most likely to have a true positive result.
SIGNS OF PROSTATE CANCER
Having to rush to the toilet to pass urine
Difficulty in passing urine
Passing urine more often than usual
Pain on passing urine
Blood in the urine or semen
Study leader, Dr Tuomas Kilpelainen, said: "I don't think routine screening should be advised until more is known on the adverse effects and costs of screening.
"If a man has urinary tract symptoms and is concerned he could have prostate cancer, the most important thing is to consult a GP or a urologist."
There is currently no organised screening programme for prostate cancer in the UK but men can request a PSA test if they want and demand is increasing.
Professor Julietta Patnick, director of the NHS Cancer Screening Programmes, said: "While the European trial, of which this Finnish study is a part, showed for the first time that prostate screening with PSA can save lives, it also suggested that 48 men would have to be treated in order to save one life.
"False positives are an issue for any screening programme, and this Finnish paper is very helpful at gaining an understanding of how they might figure in the context of prostate screening."
Results from both the European trial and a large study being carried out in the US are due this year, Cancer Research UK said.
Professor Peter Johnson, Cancer Research UK's chief clinician, said the paper showed there were "two sides" to using PSA for prostate cancer screening.
"Although for some men detecting prostate cancer early through screening can be life-saving, on the other hand the test will be abnormal for around one man in eight without cancer being detectable at that time.
"For this reason, it is important that men in their 50s and 60s can to talk to their doctor about the pros and cons of having a PSA test and only have the test if they feel it is right for them."