Does experimental pain response vary across the menstrual cycle? A methodological review

The findings on sex differences in human experimental pain research are inconsistent. One possible factor contributing to the inconsistent findings is the female hormonal cycle, as hormone levels may affect pain sensitivity. A number of studies suggest that women's responses to experimentally evoked pain vary across the menstrual cycle. However, at least an equal number of studies suggest a lack of variability. The purpose of this article is to review the literature with emphasis on what we believe could be the reasons for the inconsistent findings, namely, differences in populations sampled, timing of experimental sessions across the menstrual cycle, and nomenclature used to identify the time (phases) in the cycle when measurements were done, nature of the pain stimuli chosen, and outcomes measured. These inconsistencies and other methodological problems associated with most experimental pain studies make it difficult to draw inferences from this literature. For the science to improve, replication of significant findings using standardized timing of sessions, pain stimulus procedures, outcomes, and hormonal assessment is necessary.