“The testicles now shut off their testosterone production, and that means they are seeing a level of testosterone the same as the bloodstream,” he said. “That’s not enough to make them produce sperm.”
The problem always has been the side effects that come with these testosterone shots, Sabanegh said. They include depression, mood swings and increased libido.
This trial aimed at limiting side effects and increasing the effectiveness of the shot by combining testosterone with another sex hormone, progestogen.
“Giving testosterone alone will suppress sperm production,” Festin said. “However, the additional administration of another hormone, usually a progestin, helps increase the suppression of sperm production to lower levels, in more numbers of men.”
Researchers recruited 320 healthy men aged 18 to 45 from seven different countries across the world. The participants all had normal sperm counts and had been in monogamous relationships with female partners between the ages of 18 and 38 for at least a year.
The men received the testosterone/progestogen injections every eight weeks.
The combination lowered sperm counts to the targeted goal — 1 million per milliliter or less — in 274 of the men, researchers reported.