Higher than normal stomach pH or the presence of food slightly delays absorption. Highest concentrations are found in plasma, liver, renal cortex , heart and lung. From 50 to 80 percent of the salicylic acid and its metabolites in plasma are loosely bound to proteins. The plasma half- life of total salicylate is about 3.0 hours, with a 650 mg dose. Higher doses of aspirin cause increases in plasma salicylate half- life.
Metabolism occurs primarily in the hepatocytes.
Almost all of a therapeutic dose of aspirin is excreted through the kidneys, either as sailcylic acid or the above mentioned metabolic products. Renal clearance of salicylates is greatly augmented by an alkaline urine, as is produced by concurrent administration of sodium bicarbonate or potassium citrate. Codeine is readily absorbed from the gastrointestinal tract, and a therapeutic dose reaches peak analgesic effectiveness in about 2 hours and persists for 4 to 6 hours. The codeine plasma half-life for a 60 mg oral dose is about 2.9 hours. It passes the blood-brain barrier and is found in fetal tissue and breast milk.