In contrast to the 3-tert-butyl substituted phenothiazines and 1,2,3,4-tetrahydrocarbazoles already studied by us, the carbazole analogues lack gastric acid inhibitory properties in spite of similar Log P values. A possible explanation for this study could be that the coplanarity of the heterocyclic system with the nuclear substituents prevents these compounds from achieving a good fit with the appropriate receptor.