This multicenter, open-label study will assess the effect of multiple doses of danoprevir/ritonavir on steady-state pharmacokinetics of methadone. Subjects on stable methadone maintenance therapy (20 - 120 mg daily as single oral morning dose) will receive danoprevir 100 mg orally twice daily and ritonavir 100 mg orally twice daily for 10 days.

Inclusion Criteria:

• Male and female adults, 18 - 65 years of age, inclusive
• Subjects must be on a stable methadone maintenance regimen (20 to 120 mg/day) for the treatment of opiate addiction for at least 30 days prior to screening and should be on a stable dose for at least 14 days prior to Day -1
• Body weight >/= 50 kg
• Body mass index (BMI) 18.0 - 32.0 kg/m2
• Females of childbearing potential and males with female partners of childbearing potential must agree to use 2 forms of non-hormonal contraception during the study and for 90 days after the last study drug administration

Exclusion Criteria:

• Pregnant or lactating women and male partners of women who are pregnant or lactating
• Symptoms of methadone withdrawal at screening, on Day -2 or Day -1
• Inadequate venous access
• History or evidence of any clinically significant disease or disorder, except for drug abuse or dependence
• Positive test for alcohol or drugs of abuse at screening and up to Day -2 (with the exception of methadone)
• History (within 3 months of screening) of alcohol consumption exceeding 2 standard drinks per day on average (1 standard drink = 10 grams of alcohol); alcohol consumption will be prohibited during study confinement and for at least 48 hours before screening, dosing and each scheduled visit
• Positive for hepatitis B, hepatitis C or HIV infection