Anti-impotence drugs could offer long-term help to some men who have had prostate surgery or who otherwise lose their sexual potency, researchers reported on Monday.

A U.S. study found 30 percent of men who had a radical prostate removal procedure regained their ability to have an erection without drugs after taking Viagra for nine months.

A German study of men who took the drug every night for a year found 58 percent regained full ability to perform.

“Our study suggests that sustained, long-term rehabilitation of erectile function is possible,” Dr. Laurence Levine, a urologist at Rush University Medical Center in Chicago, who led the U.S. study, said in a statement.

Levine’s team studied prostate cancer patients who had undergone an operation called a bilateral nerve-sparing retropubic radical prostatectomy.

More than 90 percent of men who have the operation suffer from erectile dysfunction afterward, they told a meeting of the American Urological Association in San Francisco.

That could be related to nocturnal erections, which men normally have every night. No one understands why but it seems to be an exercise for the penis that, when disturbed, can result in erectile dysfunction, Levine said.

The operation may disrupt the regular nightly exercise, said Levine. Use of Viagra or similar drugs could restore it.

“The implications of this study are that physicians may have a dramatic, one-shot chance to prevent erectile dysfunction following a radical prostatectomy,” Levine said.

For the study, Levine’s group watched 54 men for 36 weeks. They were tested for nocturnal erection using a specially designed machine.

After nine months, spontaneous erectile function returned in 29 percent of the patients treated with Viagra, made by Pfizer under the chemical name sildenafil.

For the second study, Frank Sommer and colleagues at the University Medical Center in Cologne, Germany, tested 76 patients who had suffered erectile dysfunction for more than six months.

They found 58.8 percent of the patients who took Viagra every night for a year enjoyed a full return of sexual function, compared with 9.7 percent of those who only took it when they wanted to.

“After only one year, sildenafil taken regularly at bedtime may be able to bring about regression of erectile dysfunction or can be a useful tool for curing erectile dysfunction,” they told the meeting.