Laxatives are amongst the most widely used and misused drugs. Especially the "irritant chathartics" are associated with severe cell poisoning. Electrolyte disturbances like potassium deficiency may occur as a result of enhanced potassium secretion into the colon. Ultrastructural studies of patients with chronic laxative misuse demonstrate, that this functional disorder is correlated with damage of the colonic epithelial cells consisting in stereotyped lesions as changes in microvilli, reduction and structural defects of cytoplasmic organelles and an increase of lysosomes and dense bodies. The lesions have a focal distribution and may explain the potassium loss from a morphological point of view.