Choosing the unit of measurement counts: the use of oral morphine equivalents in studies of opioid consumption is a useful addition to defined daily doses.

AIM: Defined daily dose (DDD) is the most common measurement unit used in drug consumption studies. The DDD for opioids may not reflect their relative clinical potencies. The aim of this study was to explore whether opioid consumption data may be interpreted differently when adding oral morphine equivalent (OMEQ) dose as a measurement unit compared with using DDD.

METHODS: The equianalgesic ratio of each opioid relative to morphine was tabulated. Data on opioid consumption expressed in DDD were converted to OMEQs using the equianalgesic ratios. The opioid consumption was compared in three different study settings: clinical data from an opioid switching study, trends within one country and a comparison between countries.

RESULTS: Using DDD, the opioid consumption in Norway between 2004-2008 increased of 6.7%, while the increase was 23.6% using OMEQ. While DDD/1000 inhabitants/day showed that Sweden had the highest consumption of opioids among the Nordic countries, OMEQ/1000 inhabitants/day showed that Denmark had the highest consumption. In the switching study DDD indicated a reduction in analgesic dosing and OMEQ an increase when switching from WHO step II to III.

CONCLUSION: OMEQ reflects clinical dosing better than DDD, and can give additional insight into opioid consumption when combined with DDD. Using OMEQ can also lead to different conclusions in opioid consumption studies compared with using DDD alone.