Fifty-three women (19 CFS/FM patients, 16 RA patients, and 18 healthy women)
were randomly allocated to the experimental group (1 g acetaminophen) or the placebo
group (1 g dextrose). Participants underwent an assessment of endogenous pain inhibition,
consisting of an evaluation of temporal summation with and without conditioned pain
modulation (CPM). Seven days later groups were crossed-over. Patients and assessors were
blinded for the allocation.