Drug Interactions between aspirin / calcium carbonate and midazolam

MONITOR: Chronic administration of antacids may reduce serum salicylate concentrations in patients receiving large doses of aspirin or other salicylates. The mechanism involves reduction in salicylate renal tubular reabsorption due to urinary alkalinization by antacids, resulting in increased renal salicylate clearance. In three children treated with large doses of aspirin for rheumatic fever, serum salicylate levels declined 30% to 70% during coadministration with a magnesium and aluminum hydroxide antacid. Other studies have found similar, albeit less dramatic results. Antacids reportedly have no effect on the oral bioavailability of aspirin in healthy adults. However, administration of antacids containing either aluminum and magnesium hydroxide or calcium carbonate two hours before aspirin dosing led to reduced absorption of aspirin in uremic patients.

MANAGEMENT: Patients treated chronically with antacids (or oral medications that contain antacids such as didanosine buffered tablets or pediatric oral solution) and large doses of salicylates (i.e. 3 g/day or more) should be monitored for potentially diminished or inadequate analgesic and anti-inflammatory effects, and the salicylate dosage adjusted if necessary.

A number of studies have reported that antacids can delay the gastrointestinal absorption and reduce the peak plasma concentration (Cmax) of some benzodiazepines, including clorazepate, chlordiazepoxide and diazepam, although the overall extent of absorption is generally not affected. The exact mechanism of interaction is unknown, but may involve delayed gastric emptying or cation binding of the benzodiazepine. As a result, benzodiazepine onset of action may be delayed and clinical effects diminished. However, one study reported a significant increase in diazepam absorption during coadministration with aluminum hydroxide, and there was a marginal increase in the onset of sedative effect. Aluminum hydroxide also increased triazolam Cmax and systemic exposure (AUC) in 11 dialysis patients such that their drug levels reached into the range observed for the matched controls. In contrast, another study by the same group of investigators found no significant effect of aluminum hydroxide on temazepam absorption or Cmax in 11 patients with end-stage renal disease. A multi-dose study also failed to find an effect of antacids on the steady-state levels of N-desmethyldiazepam, the active metabolite of clorazepate, although an acidic environment is thought to be necessary for the rapid conversion. Based on available data, the clinical significance of this interaction appears to be minor. As a precaution, patients may consider separating the administration times of benzodiazepines and antacids or other oral medications that contain antacids (e.g., didanosine buffered tablets or pediatric oral solution) by 2 to 3 hours.