1. Concomitant treatment of desmopressin and loperamide can result in a 3-fold increase of desmopressin plasma concentrations, which can lead to an increased risk of water retention and/ or hyponatraemia.
Although very little loperamide is likely to reach the systemic circulation (see below), you should advise Mr P not to use it and recommend something else to treat the diarrhoea.