Drug-induced intestinal obstruction is uncommon but nevertheless important since failure to recognize anticoagulant ileus, for example, can lead to an unnecessary operation which carries an increased risk of mortality. In addition, a wider knowledge of these problems could, if coupled with appropriate action, reduce the incidence of intestinal obstruction. The drug-induced causes of this condition are listed in Table I according to their likely site of bowel involvement. Table 1. Drugscausing intestinal obstruction Within the lumen Within the wall Outside the wall Barium sulphate Cholestyramine Potassium salts Anticoagulants Drugs affecting smooth muscle (antihistamines, opiates, c1onidine) Drugs interfering with parasympathetic nerve transmission (autonomic ganglion blockers, muscarinic antagonists (tricyclic antidepressants» Drugs causing mesenteric vascular occlusion (contraceptive pill, adrenal corticosteroids) p-adrenoceptor antagonists (practolol) Irradiation (thorium, 198 Au) Obstruction from within the lumen Bariumsulphate The occurrence of intestinal obstruction following radiographic studies with barium sulphate has' been reviewed by Ansell (1975). Since this is not strictly drug-induced it will not be considered further. Cholestyramine This resin has recently been reported to cause intestinal obstruction in infants treated with it for obstructive jaundice (Lloyd Still 1977, Poley 1978). Potassium strictures The increasing use of diuretics, and the hypokalaemia which follows with all except spironolactone, amiloride and triamterene, has led to the widespread use of potassium supplements. Although they may be necessary in patients with heart failure who are on digitalis and diuretics concurrently, their USe in other situations is less clearly established (British Medical Journal 1977). Indeed, there is increasing evidence that for many patients their routine use is unnecessary and carries a risk of hyperkalaemia (Lawson 1974). In addition, it is well documented that potassium salts in high concentration can cause spasm of vascular smooth muscle (venous and arterial) and ischaemic ulceration of the intestinal mucosa. Subsequently, there may be haemorrhage from this site and occasionally perforation