The clinical importance of routine
drug monitoring of serum concentrations of
morphine,
morphine-6-glucuronide (M6G) and
morphine-3-glucuronide (M3G) during chronic
morphine therapy is not established. We measured
morphine, M6G and M3G serum concentrations in
cancer pain patients receiving oral (n = 263, median dose 80 mg/24 hours) or subcutaneous (sc) (n = 35, median dose 110 mg/24 hours)
morphine. Regression analyses were performed to investigate if serum concentrations of
morphine, M3G and M6G predicted
pain intensity (Brief
Pain Inventory), health-related quality-of-life variables (EORTC QLQ-C30) and cognitive function (Mini-Mental Score). Serum concentrations were also compared in patients categorized as
morphine 'treatment successes' and 'treatment failures'. We observed that serum concentrations of
morphine, M6G or M3G did not predict
pain intensity, cognitive function,
nausea or tiredness. 'Treatment failures' caused by
nausea, tiredness, cognitive failure or
constipation did not have statistically significant different
morphine, M6G and M3G serum concentrations than patients classified as 'treatment successes'. In conclusion, this study did not observe any concentration-effect relationships of
morphine, M3G or M6G with
pain intensity,
nausea,
constipation, tiredness or cognitive failure in blood samples obtained during routine clinical
drug monitoring in
cancer patients. This result suggests that therapeutic
drug monitoring as a routine tool during chronic
morphine treatment has limited value for clinical decision making.