- Use of drugs or agents within the past 2 weeks or planned use in the subsequent 4 weeks during the study period that: Alter GI transit including laxatives, magnesium or aluminum-containing antacids, prokinetic, erythromycin, narcotics, anticholinergics, tricyclic antidepressants, Selective serotonin re-uptake inhibitors (SSRI) and newer antidepressants.