Melatonin

Researchers at Hamek Medical Centre in Israel performed a six month, double-blind, placebo-controlled study of a daily treatment dose of three milligrams of melatonin in eight men.

The men consumed the melatonin or placebo orally at 6pm. Semen quality (concentration, motility and morphology), serum and seminal plasma 17-beta-estradiol (E(2), testosterone, melatonin and serum gonadotropin levels were determined after three months and at the end of the study.

In six men, there was no change in semen quality or in semen and seminal plasma hormone levels.

On the other hand, during the melatonin treatment period, two men experienced a significant drop in sperm concentration and motility (32% and 30%, respectively).

These coincided with a decline in seminal plasma and serum E(2) levels and with an increase in testosterone: E(2) ratios. Six months after the cessation of melatonin, sperm concentration and motility were normal in one man, but remained abnormal in the other one with a still elevated testosterone: E(2) ratio. Serum gonadotropin levels were unchanged during the study in all eight men.

According to the authors, "Our preliminary observations suggest that long-term melatonin administration is associated with decreased semen quality in a number of healthy men, probably through the inhibition of aromatase at the testicular level."

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Reference
Luboshitzky R, Shen-Orr Z, Nave R, Lavi S, Lavie P. (2002). Melatonin administration alters semen quality in healthy men. Journl of Andrology, 23, 572-578