Recommendations regarding how long isoniazid should be taken to prevent tub
erculosis have gradually shifted from 12 to 6 months of treatment. These re
commendations and the relevant evidence from controlled trials on duration
of treatment are reviewed. The conclusions for immunocompetent adults are:
1) 6 months of preventive treatment does not give optimal protection; 2) mo
re than 12 months of preventive treatment is not necessary; 3) 9-10 months
appears to be the optimal duration; and 4) total duration of preventive tre
atment may be more important than its continuity.