In the early 1990s, the World Health Organization sponsored the most ambitious male contraception research effort to date. The premise was simple: testosterone suppresses sperm production via the hypothalamic-pituitary feedback loop, this had been demonstrated in smaller studies since the 1970s, and it was time to find out whether this mechanism was reliable enough to actually prevent pregnancies.
What emerged from those trials was a complicated kind of success. The concept worked. The data were real. The results were good enough to call male hormonal contraception clinically feasible. And then — because this is male contraception, and things do not go smoothly — the formulation turned out to be impractical, the results varied significantly by ethnicity in ways that remain unexplained, and no pharmaceutical company moved the work forward into a marketable product.
The First WHO Trial: 1990
The WHO Task Force on Methods for the Regulation of Male Fertility enrolled 271 men across 10 centers in 7 countries on 4 continents. The regimen was straightforward: 200 mg of testosterone enanthate administered by intramuscular injection every week for up to 6 months, until the man achieved azoospermia (zero sperm detectable in ejaculate).
The 200 mg weekly dose was approximately twice what would be used for hypogonadism treatment at the time. This was a supraphysiologic dose, chosen because preliminary data suggested it was the threshold needed for reliable suppression.
The results: 65% of the 271 men developed complete azoospermia after a mean of approximately 4 months of treatment. Among those 157 azoospermic men, couples were then permitted to enter the contraceptive efficacy phase — meaning they used the weekly testosterone injections as their sole method of contraception, with no backup.
Over 1,486 person-months of contraceptive use among azoospermic men, exactly one pregnancy occurred. That is a Pearl Index of approximately 0.8 per 100 person-years — roughly equivalent to the perfect-use failure rate of combined oral contraceptive pills in women.
The reversibility data were equally encouraging. All men who completed the trial returned to normal spermatogenesis afterward. No permanent infertility was documented.
This was a genuine proof of concept. Male hormonal contraception worked — in men who achieved azoospermia, which was 65% of participants.
The Second WHO Trial: 1996
The second trial, published in Fertility and Sterility in 1996, used the same regimen — 200 mg testosterone enanthate weekly — but expanded the eligibility for the contraceptive efficacy phase. Recognizing that requiring complete azoospermia excluded too many men, the investigators allowed couples to enter the efficacy phase when the male partner reached severe oligozoospermia: initially defined as fewer than 5 million sperm/mL, later revised to fewer than 3 million/mL during the trial.
This trial enrolled 399 men across centers in Asia, Australia, Europe, and North America. Results:
- 357 men (89%) completed the suppression phase.
- Among those reaching severe oligozoospermia or azoospermia, 268 couples entered the efficacy phase.
- Over approximately 280 person-years of efficacy phase observation, 4 pregnancies occurred — 0 in the azoospermic group, 4 in the oligospermic group.
- The overall failure rate was 1.4 per 100 person-years (95% CI 0.4 to 3.7).
A failure rate of 1.4 per 100 person-years is comparable to female hormonal methods. For context, typical-use failure rates for female oral contraceptives are around 7–9 per 100 person-years. Male hormonal contraception, in men who reached adequate suppression, performed better than most couples actually use female pills.
All men in the second trial also regained normal spermatogenesis after treatment cessation.
The Ethnic Disparity Finding
The 1990 trial noted something that the 1996 trial confirmed and expanded: Asian men responded more consistently to testosterone enanthate suppression than non-Asian men.
In the 1990 trial, the proportion achieving azoospermia was higher at Asian centers. In the 1996 trial, men in Asia, Australia, Europe, and North America showed different rates of reaching the oligozoospermia threshold for efficacy phase entry. The ethnic differential was statistically significant and reproducible.
This is not a manufactured controversy or a politically convenient one. It is a documented, replicated pharmacological finding with unknown mechanism. Researchers have examined the obvious candidates — differences in androgen receptor gene polymorphisms, baseline differences in testosterone levels, differences in SHBG concentrations — without finding a clean explanation.
The practical implication is significant: a contraceptive that works in approximately 70% of Western men and a higher percentage of Asian men is not uniformly reliable. A man cannot know in advance whether he is a responder or not without testing — which requires months on the regimen, a waiting period that is impractical for a contraceptive.
Subsequent trials addressed this by adding progestins, which increased suppression rates to above 90% across more diverse populations. But the testosterone-only data from the WHO trials are the foundational layer.
The Problems: Weekly Injections and Mood Changes
Beyond the efficacy issues, the WHO trials documented two practical problems that would have killed market adoption even if the efficacy had been perfect.
Weekly intramuscular injections. Testosterone enanthate in the doses used requires injection every seven days. This is not a pill. It is not a patch. It is a needle, every week, for as long as contraceptive coverage is desired. Most surveys of men interested in male hormonal contraception find that injections rank significantly below oral and transdermal methods in acceptability. The WHO trials proved the concept. They did not prove a practical delivery vehicle.
Mood and behavioral effects. At 200 mg testosterone enanthate weekly — supraphysiologic by standard clinical definitions — some men reported mood changes, increased irritability, and alterations in libido (both directions). The WHO 1990 trial reported approximately 1.5% of participants noted mood changes and 4.4% noted libido changes as adverse events. These rates were not catastrophic by clinical trial standards. But they became part of a broader narrative about male hormonal contraception side effects that would resurface — and, in 2016, derail — a later trial in dramatically different circumstances.
What These Trials Established
The WHO testosterone trials of the 1990s established several things clearly:
- Testosterone-based suppression of spermatogenesis works as a contraceptive in men who reach adequate suppression.
- Adequate suppression is achievable in the majority of men, though not all.
- The suppression is fully reversible.
- The contraceptive efficacy in suppressed men is comparable to female hormonal methods.
- Weekly intramuscular injection of supraphysiologic testosterone is not a practical consumer product.
- Ethnic variability in suppression response is real and unexplained.
The WHO trials were, in other words, an almost-success. They proved the concept but not the product. The scientific community’s task for the next 25 years would be: find a formulation that is effective, practical, and tolerable. The next posts in this series tell the story of how well — and how unevenly — that task has been pursued.
The peculiar irony of the WHO trials is that by the early 1990s, the scientific case for male hormonal contraception had been made. The clinical feasibility was documented. What was missing was not the science. It was the investment, the formulation, and a pharmaceutical industry willing to bet that men would actually use the product. That missing piece has proven considerably harder to manufacture than the testosterone itself.
Next up: Post 3 — DMAU: The Almost Pill: dimethandrolone undecanoate, the oral male contraceptive candidate that completed Phase II trials and could theoretically be taken as a daily tablet. Why it still is not on pharmacy shelves.
