US researchers say the jab was almost 96% effective in tests on around 270 men who were using it, with four pregnancies among their partners.
However, a relatively high number developed side effects, including acne and mood disorders.
Researchers have been investigating potential for male hormonal contraceptives for around 20 years.
They have all been looking for an effective way of suppressing sperm production without causing unpleasant or unbearable side effects.
Because men constantly produce sperm, high levels of hormones are needed to reduce levels from the normal sperm count of over 15 million per millilitre to under one million/ml.
It looked at men aged 18-45 who had been in monogamous relationships for at least a year – and whose partners had agreed to take part.
The men’s sperm counts were checked at the beginning of the study, to ensure they were normal.
They then had two hormone injections (progesterone and a form of testosterone) every eight weeks, and were monitored for up to six months until their sperm count fell to under a million.
They were then asked to rely on the jab, which they continued to have at the same interval, as their only form of contraception during the efficacy stage of the study, which lasted for up to a year.
Once they stopped having the injections, they were monitored to see how quickly their sperm counts recovered.
Eight men had not recovered their normal sperm counts a year after the study ended.
The researchers stopped taking on new participants in 2011 after concerns were raised about side effects such as depression and other mood disorders, as well as muscle pain and acne.
Such side effects caused 20 men to drop out of the study and were reported by many others.
Intramuscular injections of 200-mg norethisterone enanthate combined with 1000-mg testosterone undecanoate, administered every 8 weeks.
Main Outcomes Measures:
Suppression of spermatogenesis by ejaculate analysis, contraceptive protection by pregnancy rate.
Of the 320 participants, 95.9 of 100 continuing users (95% confidence interval [CI], 92.8–97.9) suppressed to a sperm concentration less than or equal to 1 million/mL within 24 weeks (Kaplan-Meier method). During the efficacy phase of up to 56 weeks, 4 pregnancies occurred among the partners of the 266 male participants, with the rate of 1.57 per 100 continuing users (95% CI, 0.59–4.14). The cumulative reversibility of suppression of spermatogenesis after 52 weeks of recovery was 94.8 per 100 continuing users (95% CI, 91.5–97.1). The most common adverse events were acne, injection site pain, increased libido, and mood disorders. Following the recommendation of an external safety review committee the recruitment and hormone injections were terminated early.
The study regimen led to near-complete and reversible suppression of spermatogenesis. The contraceptive efficacy was relatively good compared with other reversible methods available for men. The frequencies of mild to moderate mood disorders were relatively high.
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