β-endorphin plasma levels were determined by radioimmunoassay (RIA) in patients and healthy controls (Table 2). The opioid responders had higher levels of β-endorphin (26.6 ± 3.59 fmol/ml) than the non-responders (24.7 ± 3.20 fmol/ml; p < 0.05) and the patients had lower β-endorphin levels (26.2 ± 3.57 fmol/ml) than the healthy controls (28.2 ± 4.63 fmol/ml; p < 0.01).
