Safety Study of Once a Day ART and Opiate Substitute.

Inclusion Criteria: * Ages 18 years or older * Previously documented diagnosis of HIV-1 infection: * by antibody assay (enzyme immunoassay confirmed by western immunoblot); or * positive HIV culture; or * detectable plasma HIV-1-RNA levels by reverse transcriptase polymerase chain reaction (RT-PCR). * Receiving stable opiate substitution (stable methadone level for ≥ 2 weeks prior to entry into the study) with methadone, levomethadone or buprenorphine * Either: * Antiretroviral (ARV) therapy-naïve(\*) and with: * CD4 counts \< 351 cells/µL; and/or * HIV-1 plasma levels \>= 30,000 copies/mL (\*)less than 3 months of ART for vertical transmission is considered as ARV therapy naïve. * Or restarting ART after treatment discontinuation with no evidence of prior HIV virological failure (virological failure defined as 2 consecutive measurements 4 weeks apart with viral load of HIV RNA \> 400 copies/mL while on ART) * Or currently receiving stable ART therapy and with virological suppression (\< 400 copies/mL), for at least 6 months and: * suffering from adherence problems because of dosing of current ART; or * suffering from side effects on the current recorded ART. * Able to give informed consent * In the opinion of the investigator is likely to be able to complete the study Exclusion Criteria: * Need for antiretroviral therapy which is not according to protocol * Pregnant or breastfeeding women * Females of childbearing potential not willing to use a barrier method(s) of contraception during heterosexual intercourse during the duration of the study * Contraindication to use of tenofovir DF 300 mg or another concomitant medication * Known hypersensitivity to the active component or excipients * Prior receipt of tenofovir * Evidence of clinical, genotypic, or phenotypic resistance to any ARV * History of virological failure while on previously recorded ART regimens (virological failure defined as 2 consecutive measurements 4 weeks apart with viral load of HIV RNA \> 400 copies/mL) * Acute, life-threatening infection or malignancy that needs systemic therapy * Any clinical laboratory findings obtained during screening that could be a risk factor for the patient during the study: * Grade 4 increase of any laboratory value * Grade 3 (\> 5-10 upper limit of normal \[ULN\] increase in transaminases) at the screening visit * Any other clinical condition or prior therapy that, in the opinion of the investigator, would make the patient unsuitable for the study or unable to comply with the study requirements. * Current use of medication that, in the investigator's opinion or sponsor's opinion, will interfere with the study medication * Participation in other clinical trials * More than three months of ART treatment for vertical transmission prophylaxis * Current receipt of adefovir dipivoxil