Unreliable drug histories, analgesics, and changes in renal function.

One hundred and nine outpatients were questioned on their use of analgesics and asked to provide urine and blood samples. Forty-one per cent said they used analgesics daily and 54% said they had ingested analgesics during the 3 days before questioning. The analgesics used were mainly aspirin or paracetamol, alone or in combination. Five (42%) of the patients who denied the use of analgesics, and 10 (29%) of those who denied intake during the preceding week, had detectable amounts of paracetamol or aspirin in their urine. Mean serum urea, although still within the normal range, increased with increasing use of analgesics. The mean serum urea of patients who used aspirin and paracetamol in combination was significantly higher than that of those who used either of the drugs alone. The findings suggest that one should not rely heavily on the history of analgesic use, and that by replacing phenacetin with paracetamol one might not have achieved much as far as renal function is concerned.