Objective: To investigate whether establishment and maintenance of chronic opioid blockade throughout the follicular phase of the menstrual cycle influences midcycle and luteal phase prolactin levels.
Design: Randomized, double-blind, crossover study.
Setting: Academic research environment.
Patient(s): Volunteers, aged 21-35 years, with regular menstrual cycles.
Intervention(s): Naltrexone (50 mg) or placebo were administered on cycle days 2-14. Blood samples were obtained in the early follicular phase and in the periovulatory and midluteal phases of the menstrual cycle.
Main outcome measure(s): Serum prolactin levels.
Result(s): In the early follicular phase, serum prolactin levels were equivalent in naltrexone (12.0 +/- 2.7 microgram/L; mean +/- SE) and placebo (12.1 +/- 2.9 micrograms/L) cycles. A statistically significant increase in serum prolactin was observed on the day of the LH surge (naltrexone: 22.6 +/- 3.7 micrograms/L; placebo: 21.7 +/- 2.7 micrograms/L; P < 0.05 versus early follicular phase), but no difference between treatments was observed. However, midluteal prolactin levels were statistically significantly lower in naltrexone cycles compared with placebo cycles (12.6 +/- 3.3 versus 15.4 +/- 3.0 micrograms/L; P < 0.05).
Conclusion(s): Chronic blockade of opioid activities during the follicular phase does not affect midcycle prolactin increments, but withdrawal of opioid blockade may enhance opioid effects on prolactin levels in the luteal phase.