Effects of reserpine on prolactin levels and incidence of breast cancer in postmenopausal women.

Epidemiological studies of reserpine use and breast cancer have generally found only small increases in breast cancer risk, even after long-term use. Prolactin levels in short-term reserpine users have been reported to be in the range of those of lactating women, levels which rodent experiments suggest should greatly increase breast cancer incidence. We measured prolactin levels in 15 women who had been taking reserpine-containing drugs for at least 5 years and compared them to levels in 15 women taking non-reserpine-containing antihypertensives and 15 women taking no antihypertensive medicines. Although reserpine users had significantly elevated levels of prolactin, their mean level was only approximately 50% greater than the mean level of the combined results from the two control groups. Based on a statistical model of breast cancer incidence, we calculate that such increases in prolactin in the postmenopausal period would be likely to cause only small increases in breast cancer risk, as have been observed in epidemiological studies.