Relationship between the acetylator phenotype, plasma sulfapyridine levels and adverse effects during treatment with salicylazosulfapyridine in patients with chronic bowel diseases.

We examined 122 patients with inflammatory bowel disease treated with salicylazosulfapyridine. Forty-two (34.5%) had adverse effects that led to discontinuation of therapy in 14 (11.5%). In 33 patients the effects appeared to be dose dependent. The plasma sulfapyridine levels in patients exhibiting gastrointestinal side effects were significantly higher than in patients with no adverse effects (41.0 +/- 20.3 and 23.8 +/- 14.8 micrograms/ml respectively, P less than 0.001). Plasma sulfapyridine levels were significantly higher in slow than in fast acetylators (31.5 +/- 17.1 vs. 22.2 +/- 17.1 micrograms/ml). Slow acetylators had three times as many side effects as fast acetylators; however this difference was not statistically significant.