In 1985 the Committee for Safety of Medicines warned that cotrimoxazole was likely to cause adverse effects, particularly bone marrow suppression and death in patients over 65 years. Fears regarding the safety of cotrimoxazole received media attention in 1994 and by 1995 the indications for use of cotrimoxazole were restricted. We studied the prescribing patterns of cotrimoxazole and trimethoprim within the state-supported General Medical Services (GMS) scheme in Ireland to document the influence of media and regulatory advice on prescribing patterns. The decline in prescribing of cotrimoxazole (by some 70,000 prescriptions within a year) preceded the change in its licensed indications emphasizing the strong influence of media reports on prescribing patterns. At present the prescribing rates of cotrimoxazole appear greater than is warranted and is similar among children (18/1000 patients) and the elderly (15/1000 patients) which is in contrast to the prescribing rates for trimethoprim in children (17/1000 patients) and the elderly (101/1000 patients). The most common length of therapy for both antibiotics was 7 days despite evidence that shorter courses of these antibiotics are suitable for treating uncomplicated urinary tract infections. Cotrimoxazole is still being prescribed to a significant degree across all age groups despite its revised indications. The need for a continuous input by regulatory agencies is clear. Copyright (c) 2000 John Wiley & Sons, Ltd.