Table 2.

Drug interactions potentially involving inhibition of CYP450s by INH

Drug affectedCYP450(s) involvedType of study and subjectsPharmacokinetic changes observedaSide effects observedINH dose (mg/day)Reference(s)
PhenytoinCYP2C9, CYP2C19Several studies and case reportsIncreased levels in serum, more in SAAtaxia, hyperreflexia, nystagmus, tremor300–600 3, 24,30
CarbamazepeineCYP3A, CYP2C8Case reportIncreased levels in plasmaDrowsiness, ataxia, nystagmus, headache, vomiting200–300 54, 58, 59
DiazepamCYP3A, CYP2C19Randomized, crossover designNo change in Vd , increasedt 1/2 (33%), decreased CL (26%), more effect in SASedation, respiratory depression180 36
TriazolamCYP3A, CYP2C19Placebo controlledIncreasedt 1/2 (32%)None reported180 37
PrimidoneUnknownCase study, femaleIncreasedt 1/2 (60%), increasedC ss (83%), decreased plasma PB and PEMA levelsNone reported300 48
EthosuximideCYP3ACase reportIncreased level in serumIncreased psychotic behavior300 55
Vitamin DVitamin D hydroxylaseParallelDecreased 1,25(OH)2D, increased PTH, decreased Ca2+ and PO4None reported300 4
AntipyrineCYP1A2, CYP3A, CYP2C19ParallelDecreased CL (18%)None reported300 4
Chlorzoxazoneb CYP2E1Placebo controlledIncreased or decreased CL, more effect in SA than in FANone reported300 27
Acetaminophenb CYP2E1, CYP1A2Placebo controlledIncreased or decreased CL, more effect in SA than in FAIncreased hepatotoxicity?300 61
Isoflurane and enfluraneCYP2E1Placebo controlledIncreased defluorination, more effect in SA than in FADecreased effectiveness300 46
TheophyllineCYP1A2, CYP3A, CYP2E1Placebo controlled, case reportNo change or decrease in CL (16 to 23%)Seizure, nausea and vomiting, palpitation300–600 46
VincristineCYP3ACase reportNATingling of fingers, weakness in limbs300 7
WarfarinCYP2C9Single case reportNAIncreased PT and bleeding600 46
  • a PB, phenobarbital; PEMA, phenylethylmalonamide; Vd , volume of distribution; t 1/2, terminal elimination half-life; CL, clearance; SA, slow acetylators; FA, fast acetylators; NA, not available; PTH, prothrombin time; 1,25(OH)2D, 1,25-hydroxy-vitamin D.

  • b Changes in CL depend on the time after INH administration.