No interaction between H2 blockers and isoniazid.

Antacids and anticholinergic drugs may delay gastric emptying and cause reduced absorption of isoniazid, but the influence of reduced gastric acidity has not been determined. In the present investigation, the influence of cimetidine and ranitidine on uptake and elimination of isoniazid was determined in six male and seven female healthy volunteers. The group comprised five rapid and eight slow acetylators, similar to the proportions in the general population. H2 blockers did not delay or reduce uptake of isoniazid, which thus seemed to be pH independent. Neither the acetylation of isoniazid nor the oxidative metabolism of acetylated isoniazid appeared to be affected by cimetidine or ranitidine.