Antacid attenuates the laxative action of magnesia in cancer patients receiving opioid analgesic.

Objective: This study was designed to investigate pharmacological interaction between magnesium laxative and antacid in patients receiving opioid analgesic.
Methods: Data obtained from a total of 441 eligible patients receiving opioid analgesic for the first time were retrospectively analysed. The incidence of constipation, defined as stool-free interval of 3 days and more within the first week of opioid intake, was compared between patients who took laxative alone and those who received laxative in combination with antacid.
Key Findings: Laxatives were prescribed in 74% of patients, among them 61% received antacids such as proton pump inhibitor and H2 receptor blocker. Magnesia was the most commonly used laxative (89%). Constipation occurred in 21% and 55% of patients with and without laxatives, respectively. Antacids reversed the laxative action of lower doses (<2000 mg/day) but not higher doses (>2000 mg/day) of magnesia without affecting the effects of other laxatives. Therefore, it is suggested that both acid-dependent and acid-independent mechanisms may operate in the laxative action of magnesia, in which the former may be involved in the action of lower doses of magnesia.
Conclusion: Care should be taken to avoid the unfavourable pharmacological interaction between low doses of magnesia and antacid.
(© 2016 The Authors. Journal of Pharmacy and Pharmacology published by John Wiley & Sons Ltd on behalf of Royal Pharmaceutical Society.)