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What Is Testosterone Undecanoate?

By Maggie J. Hall
Updated May 17, 2024
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Testosterone undecandoate might be prescribed for the treatment of hypogonadism in men, but more recently, the androgen has been used as a male contraceptive. Since the 1980s, researchers have been testing testosterone variations in an attempt to develop one. While this testosterone ester produces effective results in Asian men, the hormone requires the addition of other hormones to produce equally successful outcomes for men in other countries.

This ester of testosterone suppresses follicle stimulating hormone (FSH) and luteinizing hormone (LH), interfering with the signals required by the testes to produce sperm and resulting in lowered sperm counts. Men usually do not experience the full effects of treatment, however, until after the first two or three months of receiving the drug. Sperm counts then remain low for the duration of treatment. Counts generally return to normal anywhere from two to six months after cessation of treatment.

Chinese researchers first combined 200 milligrams (mg) of testosterone undecandoate with tea tree oil and administered the formula by intramuscular injection. Men received periodic injections over the course of 30 months. During the first trials, 80% to 90% of the men involved in the study developed sperm counts of less than one million sperm per milliliter (ml) of ejaculate. Normal sperm counts average around 20 million sperm per ml. After three months of treatment, the male contraceptive was 90% to 100% effective.

Later tests involved 308 men who were dosed with 500 mg of testosterone undecanoate every 12 weeks. The first sperm samples indicated that 299 men had counts below 3 million per ml. After one year, out of 296 men, only one man had impregnated his wife. Studies involving men in other countries were less successful and generally demonstrated only 60% efficiency.

Researchers proposed chemical and genetic mechanisms to explain why only Asian men respond well to testosterone undecanoate therapy but have been unable to produce a conclusive explanation. Testosterone undecandoate has been combined with various forms of of progestin. Non-Asian men received 1,000 mg injections of testosterone every six weeks. They also received 200 mg of norethisterone every six weeks. After 32 weeks of treatment, 13 out of 14 males were azoospermic, or without sperm.

Testosterone undecanoate may be administered by injection, orally, or by transdermal patch. It can also be applied topically, in gel form, or as small pellets implanted under the skin. The patch was the least popular method, as men often experienced itching, redness, and blistering. The hormone was also not absorbed as readily using this method and provided inferior protection.

Side effects of testosterone undecanoate, with or without progestin, include headache, slight weight gain, and depression. Men might also experience male pattern baldness and oily skin. Lab results show that men may experience a 10 to 20 percent decrease in high-density lipoproteins (HDLs) but an increase in hemoglobin, hematocrit, and prostate specific antigen (PSA) levels.

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