The study lasted for seven years. When it began, the men completed a survey about their sexual function.
Survey topics included the men's ability to have an erection when desired, degree of satisfaction with sexual activities, change in sexual performance, frequency of sexual activities, age, medical conditions, and smoking status.
Survey scores could range from zero to 100. Higher scores indicated worse sexual function.
The men repeated the survey at least one other time during the seven-year study. During that time, they were given either Proscar or placebo pills without knowing the difference.
Proscar Study's Results
Proscar "increased sexual dysfunction only slightly and its impact diminished over time," write Moinpour and colleagues.
Six months after the men started taking their assigned drugs, the survey scores were about three points higher for the men taking Proscar. By the end of the study, that gap narrowed to about two points.
Sexual problems can happen for many reasons. For instance, the researchers note that getting older worsened survey scores by about eight points during the study. That makes Proscar's impact look "small," write Moinpour and colleagues.
However, the researchers note that the data only include men who stuck with the study -- not those who quit taking their assigned drugs.
"Sexual dysfunction was often cited as the reason for dropping out," Moinpour's team notes, adding that there were more dropouts taking Proscar than the placebo.
However, the researchers argue that dropouts probably didn't affect the study's findings.
Proscar is made by the drug company Merck, which provided Proscar and the placebo for the study. Merck also gave the researchers small grants to promote study participation and adherence.