Codeine and morphine blood concentrations increase during blood loss

During extensive blood loss, a plasma volume refill will take place by transfer of extravascular fluid into the circulation. Drugs present in this fluid may follow and cause a rise or a drop in blood drug concentration, depending on their levels and accessibility in the restoration fluid. This study explored the possible changes of codeine, and its metabolite morphine, in whole blood during a standardized exsanguination in the rat. Three doses containing 5 mg codeine were given orally. In eight rats, blood loss was accomplished by slowly withdrawing 0.8 mL blood at 10 min intervals during 70 min. In control rats, blood was withdrawn only at 0 and 70 min. At 70 min, the final/initial codeine and morphine concentration ratios were 0.70 +/- 0.38 and 0.88 +/- 0.47, respectively, in controls, but increased to 1.28 +/- 0.44 (p = 0.014) and 1.41 +/- 0.34 (p = 0.021), respectively, in exsanguinated rats. It is concluded that blood loss can affect blood drug concentrations.