This is a study on the utility of the modification of doses of efavirenz guided by its plasma concentration (therapeutic drug monitoring) in HIV-infected patients initiating treatment with Sustiva.

Currently, efavirenz is dosed systematically, without taking into account the individual characteristics of each individual patient. However, plasma concentration of efavirenz may widely vary between different subjects that receive the same dose of the drug (interindividual variability).

Therapeutic drug monitoring (TDM) signifies individualised pharmacological dosing, based on the plasma levels that each patient presents. This strategy has been broadly used in the field of the treatment of other medical conditions and is acquiring growing interest in the field of antiretroviral treatment. Thus, the use of TDM for the treatment of naïve patients with nelfinavir or with indinavir has translated into an increase in the proportion of individuals with suppressed viral load and also into a reduction in HAART-induced adverse events . However, data on the utility of the therapeutic monitoring of the levels of efavirenz in HIV-infected patients are very scant.

On the basis of the above, it might be thought that the modification of the doses of efavirenz, guided by its plasma concentration, in patients receiving this drug and whose plasma levels of efavirenz are outside the therapeutic range, might improve the tolerability of the treatment without compromising virological efficacy.

Inclusion Criteria:

• HIV-1 patients.
• According to the criteria of the treating physician, the need to initiate a regimen of antiretroviral treatment that includes efavirenz (both antiretroviral-naive patients and others on treatment with protease inhibitors switching to efavirenz for salvage reasons or for simplification of the anti-retroviral therapy are included).
• Absence of opportunistic infections and/or tumours in the three months prior to inclusion.

Exclusion Criteria:

• History of allergic hypersensitivity to the investigational drug.
• History of previous failure with antiretroviral treatment with non-nucleoside reverse transcriptase inhibitors or previously documented resistance to efavirenz
• History of psychiatric comorbidity which, in the investigator's opinion, renders the use of efavirenz inadvisable.
• Active consumption of alcohol (>50 g/day) or other illegal drugs (except cannabis)
• Suspicion of unsuitable compliance with the antiretroviral treatment.
• Pregnant women or breast-feeding mothers.