Prolactin responsiveness to repeated decremental doses of sulpiride

The variation in the prolactin response to sulpiride was studied in six normal men by repeating the same dose of the drug (50 mg) after 24 hours and on three subsequent occasions, repeating this 2 day test at an interval of 6 days with progressively halved doses of sulpiride. A similar PRL response occurred on the first day of each test period and the peak response was highly significant (P less than 0.001), occurring within 30 minutes. A gross blunting of the PRL response on the second day of each period was noted throughout the study. The difference in delta PRL between the first and second day of each period was highly significant (P less than 0.001). The delta PRL increment on the second day was inversely proportional to the dose of sulpiride; the differences in delta PRL between periods 1 and 3 and periods 1 and 4 being highly significant (P less than 0.001). This study suggests that a much lower dose of sulpiride than that normally used is adequate to stimulate PRL secretion and that care must be taken in the timing of repeat testing.