Of those men, 266 remained in the study to see if the shots would reduce the risk of pregnancy.
“It produced a pregnancy rate of 1.57 per 100 in continuing users, which is comparable to other reversible contraceptive methods, which presently are used by women,” Festin said.
However, Sabanegh questioned whether the targeted sperm reduction goal was low enough to be truly effective in widespread use.
“We know that people can routinely cause pregnancies with sperm counts under a million,” he said. “The four pregnancies that were caused were in patients who had sperm counts under a million.”
Dr. Elizabeth Kavaler, a urology specialist with Lenox Hill Hospital in New York City, agreed with Sabanegh.
“The sperm count goal is not that low. It should be lower,” Kavaler said. “I wonder if they had 50,000 patients in the trial, where their pregnancy rates would be.”
There also were 771 incidents of side effects assessed to be likely or definitely related to the use of the birth control shot, the findings showed. The most common were acne, increased libido, muscle pain, and mood and emotional disorders.
Twenty men dropped out of the study due to side effects, and the adverse events eventually led to the early end of the study, Festin said.