>Weekly injections of testosterone reduce the sperm counts of almost all men to below the threshold needed for conception, according to a study by the World Health Organization.
The two-and-a-half-year study of 399 couples in nine countries found that the injections of the male sex hormone were an effective contraceptive for 98.6 percent of the participants, according to the report in the current issue of Fertility and Sterility, the journal of the American Society for Reproductive Medicine.
Testosterone was already known to inhibit the production of pituitary signals that normally instruct the testes to produce sperm.
The men, 21 to 45 years old, were all normal, healthy, fertile and in stable relationships. They received weekly testosterone shots in the buttocks. There were only four pregnancies among couples in the study during the test period.
Another result of the study was the discovery of the sperm threshold necessary for contraception, according to Dr. Fred Wu, a senior lecturer in endocrinology at Manchester University, who supervised the monitoring of 72 British couples.
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Earlier research had shown that testosterone injections would virtually eliminate sperm production in 60 percent of men tested. The problem was in inhibiting contraception among men whose sperm level was lowered, but not wiped out.
The study showed that a sperm count of less than three million was enough to guarantee no conception. That threshold was met by a further 38.6 percent of the men, bringing effectiveness to 98.6 percent.
Dr. Wu "This means the vast majority of men can use it," Dr. Wu said.
He said it was still not known why the injections were not effective in some men.
One American expert said the study might point to a breakthrough in contraceptive techniques.
"It would seem a strong indication that male hormone given by injection could be an effective contraceptive, the only drawback being the painful method," said Richard Sherins, director of andrology at the Genetics and IVF Institute in Fairfax, Va. "It would show that testosterone, like estrogen in a female, can be an additional method of reproduction control."
Dr. Wu estimated it could take five to eight years before a better technique of administering the method was perfected.
"It is a painful injection in the buttock," Dr. Wu said. "But we believe we can improve on that with long-acting injections, tablets, patches and even pellet implants."
In addition to Britain and the United States, the countries participating in the study were Australia, China, Sweden, France, Hungary, Singapore and Thailand.
The American tests were carried under the auspices of scientists at the Veterans Affairs Medical Center in Seattle and at the U.C.L.A. Medical Center in Torrance, Calif.