A study of a vaginal progesterone ring that released almost 10 mg/day progesterone and maintained mean progesterone levels of 4.4 ng/mL (range 2.4–6.5 ng/mL) found that ovulation was inhibited in some but not all women. In another study, a progesterone vaginal ring that released about 10 mg/day progesterone and produced progesterone levels of around 4 ng/mL (range 3–5.2 ng/mL) resulted in ovulation occurring in 25% of treated breastfeeding women compared to a rate of 56% in a control group of breastfeeding women. A study in rhesus monkeys found that a vaginal ring delivering 0.235 or 1.77 mg/day progesterone inhibited ovulation in all monkeys at the higher dose and in a proportion of monkeys at the lower dose. A dose of progesterone of 5 to 10 mg/day by intramuscular injection has been found to prevent ovulation in women and has been considered effective as a progestogen-only injectable contraceptive. Short-term therapy with 300 mg/day oral progesterone had no effect on luteinizing hormone pulse frequency in women. Treatment with a high dosage of oral progesterone of 100 mg four times per day (or 400 mg/day total) in men for 10 days did not cause any change in testosterone levels, suggesting that oral progesterone has little or no antigonadotropic effect in males at typical clinical dosages. In addition, a study found that administration of 1,000 mg/day oral progesterone for 3 months had no significant effect on urinary gonadotropin excretion. On the other hand, a single 50 mg intramuscular injection of progesterone, which is associated with high progesterone levels of approximately 50 ng/mL (or early- to mid-pregnancy levels), resulted in substantial (50–60%) suppression of luteinizing hormone, follicle-stimulating hormone, and testosterone levels in men. Similarly, continuous or intermittent intravenous injections of 100 to 400 mg/day progesterone for 10 days significantly decreased urinary gonadotropin excretion. Progestogens in general are able to suppress gonadal testosterone production in men by a maximum of about 70 to 80% or to just above castrate levels when used at sufficiently high doses. A study using 50 mg/day progesterone by intramuscular injection in five men found that the medication produced azoospermia or severe oligozoospermia in all within 10 weeks, suppressed libido, erectile function, and ejaculate volume to minimal levels, produced slight gynecomastia in two of the men, moderately decreased testicular size, and impaired testicular morphology.