We reported in June on a case series of patients experiencing serious cardiovascular problems, including arrhythmias, attributed to high doses of loperamide. This has occurred largely due to the fact that loperamide has been abused as an opioid substitute. 

The FDA has subsequently issued a warning about this. They advise not exceeding the recommended dose and also provide examples of drugs that can potentially interact with loperamide, increasing loperamide levels. These drugs include cimetidine, clarithromycin, erythromycin, gemfibrozil, itraconazole, ketoconazole, quinidine, quinine, ranitidine and ritonavir.