Drugs commonly used for symptom relief or healing of peptic ulcers can be divided into 4 main groups: antacids (including bismuth for convenience), anticholinergics, H2-receptor antagonists and liquorice derivatives. For all such drugs the safety margin during normal clinical use must be great, as they are widely used in otherwise healthy subjects. Side effects of the widely used insoluble antacids usually take the form of bowel upset: constipation and diarrhoea from aluminium- and magnesium-containing preparations, respectively. Accumulation of individual ions is uncommon, but aluminium encephalopathy may occur in the presence of severe uraemia. Although bismuth may be absorbed from some preparations, encephalopathy has not been reported with the popular form tripotassium-dicitrato-bis-muthate. Experimental single dose studies have shown altered disposition of many drugs when taken with antacids; clinically important interactions during long term therapy seem to be rare. Anticholinergic drugs taken in doses which reduce gastric acid secretion produce the expected side effects of dry mouth, constipation and urinary retention. Although interference with the accommodation reflex can be troublesome, the danger of precipitating acute glaucoma has probably been overstated. The development of selective muscarinic antagonists should provide antisecretory agents with minimal side effects. The H2-receptor antagonist cimetidine has been in widespread clinical use for 5 years, and serious adverse reactions seem rare. The unwanted effects most commonly seen are gynaecomastia when used alone in high doses, confusion in the elderly or those with impaired excretory mechanisms, and modest inhibition of the oxidative metabolism of other drugs. Overall the drug has proved remarkably safe. In practice the only significant adverse effect encountered with carbenoxolone is salt retention and hypokalaemia. Although identified quite frequently, this is seldom severe enough to warrant cessation of therapy. The elderly and those with pre-existing abnormalities of salt and water handling are at greater risk, and the drug should be avoided in these patients.