Inclusion Criteria:

1. Patients of either sex aged > 18 years.
2. HIV positive.
3. Stable antiretroviral therapy consisting of efavirenz (EFV) given with Combivir® or zidovudine (AZT) + lamivudine (3TC) for at least 6 months.
4. Patients with viral loads < 50 copies/ml on last 2 consecutive tests and < 400 copies/ml for > 3 months.
5. Patients requiring a lipid lowering agent must be established on a stable dose/frequency for at least 12 weeks prior to Baseline and be expected to continue on stable dose/frequency for the duration of the study.
6. Negative serum pregnancy test (females of childbearing potential only).
7. Willingness to use effective contraception (such as barrier or coil methods) by both males and females while on study treatment and for 30 days following study drug completion.
8. The ability to understand and sign a written informed consent form, which must be obtained prior to initiation of study procedures.

Exclusion Criteria:

1. Pregnant or lactating female.
2. History of AZT monotherapy.
3. Use of anabolic steroids, with the exception of testosterone for documented hypogonadism, within 90 days prior to the Baseline visit.
4. Documented parvovirus infection.
5. Use of erythropoietin within the last six weeks.
6. Patients who have had a blood transfusion in the last six weeks.
7. Karnofsky score < 50.
8. Prior history of significant renal disease.
9. Prior history of osteopenia/osteoporosis.
10. Creatinine clearance < 60mL/min.
11. AST/ALT > 5 x upper limits of normal (ULN).
12. Previous adefovir dipivoxil or cidofovir therapy.
13. Known history of resistance (including primary resistance) to any of the study medications - tenofovir disoproxil fumarate (TDF), emtricitabine (FTC), AZT, 3TC, or EFV.

14. Patients receiving ongoing therapy with any of the following (administration of any of the following medications must be discontinued at least 30 days prior to the Baseline visit and for the duration of the study period):

    • Nephrotoxic agents
    • Probenecid
    • Systemic chemotherapeutic agents (i.e. cancer treatment medications)
    • Systemic corticosteroids
    • Interleukin 2 (IL 2)
    • Drugs that interact with efavirenz
    • Dihydroergotamine
    • Ergotamine
    • Midazolam
    • Triazolam
    • Cisapride
    • Rifampin
    • Ergonovine
    • Methylergonovine
15. Patients with known hypersensitivity to any of the study medications or excipients.
16. Active, serious infections (other than HIV-1 infection) requiring parenteral antibiotic therapy within 15 days prior to Screening.
17. Patients who are currently taking part in any other clinical trial or have taken part in a clinical trial of a new chemical entity within 1 month prior to Screening.
18. 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 dosing requirements.
19. Patients with cancer (except basal cell carcinoma).
20. Co-infection with hepatitis B virus