Up to 89 percent of the men who received the combination formulas saw their sperm concentrations plunge to less than 1 million sperm per milliliter, versus just 23 percent of those receiving only testosterone.
Normal sperm concentration is more than 15 million sperm per milliliter, according to the Mayo Clinic's website.
Up to 78 percent of men receiving the drug combination in the study saw their concentrations drop to no sperm at all, versus only 23 percent of men taking testosterone alone.
The testosterone/progestin combination works by shutting off the hormones that control production of sperm in the testes, Wang explained.
One important and unanswered question is what the long-term side effects of this regimen might be.
The answer, said Wang, is "We just don't know."
That's apart from mild-to-moderate acne or an increase in acne, which was seen in about 21 percent of participants.
A new formulation currently being developed that contains half the amount of testosterone might decrease or eliminate that problem, Wang speculated.
Another question is how reversible this would be in terms of restoring sperm counts, Alukal said.
In addition to planning more studies on this combination protocol, Wang is also in early phases of testing the male hormone dimethandrolone, which is more potent than testosterone.
Because the new study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
There's more on contraception at the American College of Obstetricians and Gynecologists.
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