In drugs-of-abuse testing, opiates constitute a delicate and controversial task because morphine may occur in urine for several reasons. Ethylmorphine (EtM), which is used as an antitussive drug in many countries, is metabolically converted to morphine. The present study was performed in order to document intra- and interindividual differences in morphine formation after single-dose intake of EtM at two different doses (25 and 50 mg). The urinary excretion of opiates was measured during 48 h with EMIT and gas chromatography-mass spectrometry in 10 healthy volunteers. The mean values of totally recovered EtM and morphine in hydrolyzed urine during 48 h were 42 and 47% of the given dose at high and low dose level, respectively. The ratio between total recovered morphine and EtM ranged from 19 to 131% with a mean value of 48%. The rate of positive outcome in the EMIT opiate-screening assay was 100% during the first 24 h for both doses, and it was still high (> or = 67%) in the 24-48 h time interval. It was found that the decline in urinary EtM is more rapid than for morphine, which leads to an increasing morphine/EtM ratio in urine over time. THe mean value of the morphine/EtM ratio was found to be greater than 1 during the 12-24 h interval and finally increased to greater than 10. There was an intra-individual concordance in morphine/EtM ratio between doses, but there was marked interindividual variation. Morphine/EtM ratios that were greater than 1 were only seen when the concentration of morphine was below 300 micrograms/mmol creatinine. Our results demonstrate that morphine is formed from EtM at a high and variable rate and may be present in urine in concentrations greater than those of EtM even shortly after drug intake.