Relationship of plasma salicylate levels to pain relief with two different salicylates.

In a preliminary open study of salsalate (3 g daily for 4 weeks) in 61 patients with rheumatoid arthritis or osteoarthrosis, it was found that although the drug produced satisfactory analgesia in 64% of patients, the incidence of side-effects was high (57% of patients): most were symptoms of salicylism and probably related to the high plasma salicylate levels achieved. In a second open study, 20 patients with osteoarthrosis were treated for 4 weeks with 250 mg diflunisal twice daily and then crossed over to salsalate (3 g daily) for a further 2 weeks. The results of subjective assessments of pain relief showed that both drugs produced satisfactory analgesia, and neither was associated with a significant level of gastro-intestinal bleeding. During the diflunisal treatment period there were no reports of salicylism, and plasma salicylate levels were very much lower than those measured after salsalate. The pain relieving effects of both drugs, assessed from patient preference for one or the other treatment, were unrelated to the plasma salicylate levels and it is suggested that plasma levels may have more relationship to the incidence of side-effects than with therapeutic effects.