The renal clearance is greatly augmented by an alkaline urine as is produced by concurrent administration of sodium bicarbonate or potassium citrate. The biotransformation of aspirin occurs primarily in the hepatocytes. The bioavailability of the aspirin component of Butalbital compound with Codeine capsules is equivalent to that of a solution except for a slower rate of absorption. Codeine Codeine is readily absorbed from the gastrointestinal tract. It is rapidly distributed from the intravascular spaces to the various body tissues, with preferential uptake by parenchymatous organs such as the liver , spleen, and kidney.
Codeine crosses the blood-brain barrier, and is found in fetal tissue and breast milk.
Codeine is not bound to plasma proteins and does not accumulate in body tissues. The plasma half-life is about 2.9 hours. The remainder of the dose is excreted in the feces. At therapeutic doses, the analgesic effect reaches a peak within 2 hours and persists between 4 and 6 hours. The bioavailability of the codeine component of Butalbital compound with Codeine capsules is equivalent to that of a solution.