PIP: The fact that pregnancy lowers long-term basal prolactin secretion has lead to the suggestion that low prolactin levels, which by inference may occur after pregnancy at an early age, may protect against breast cancer. There is evidence that prolactin is a co-carcinogen in rodent mammary cancer, but aspects of rodent mammary gland biology may not be applicable to humans. Research in primates suggests that some factor from the pituitary is required for breast cancer, although it is neither prolactin nor growth hormone. similarly, one experiment has been reported that some pituitary factor may potentiate estrogen in promoting growth of human breast cancer cells in cell culture. Treatment of human breast cancer with prolactin-lowering drugs, however, is ineffective. Removal of the pituitary os effective in treating human breast cancer, and furthermore, persons without mature breast tissue, such as women with Turner's syndrome or men, have a very low incidence of breast cancer. Overall, the evidence linking prolactin to breast cancer is insufficient to warrant use of prolactin-lowering ergot drugs as a preventive measure in those at risk of breast cancer.