Absorption Absorption from the gastrointestinal tract is affected by Gastric acid secretion Bile salt formation Gastric emptying time Intestinal motility Bowel length and effective absorptive surface Microbial flora Illness All these factors vary with age (1). Reduced gastric acid secretion increases bioavailability of acid-labile drugs (eg, penicillin) and decreases bioavailability of weakly acidic drugs (eg, phenobarbital).