Healthy men should no longer receive a P.S.A. blood test to screen for prostate cancer because the test does not save lives over all and often leads to more tests and treatments that needlessly cause pain, impotence and incontinence in many, a key government health panel has decided. The draft recommendation, by the United States Preventive Services Task Force and due for official release next week, is based on the results of five well-controlled clinical trials and could substantially change the care given to men 50 and older. There are 44 million such men in the United States, and 33 million of them have already had a P.S.A. test, sometimes without their knowledge, during routine physicals. The task force's recommendations are followed by most medical groups. Two years ago the task force recommended that women in their 40s should no longer get routine mammograms, setting off a firestorm of controversy. The recommendation to avoid the P.S.A. test is even more forceful and applies to healthy men of all ages.