This study will compare two different medications, used alone or in combination, to prevent relapses of HE. Lactulose is a laxative (causes increased bowel movements) which has been widely used in the treatment of HE. However, it is not clear how effective it is for this purpose and is not always well tolerated. Rifaximin is an oral antibiotic which has been studied in the treatment of HE, including longer treatment courses but its role in the prevention of HE relapses is not clear. No studies to date have compared the use of lactulose to combination treatment with lactulose and rifaximin in the prevention of recurrent episodes of HE.