Sustiva and Norvir shouldn’t be combined without close monitoring of the patient’s condition. Active ingredients in the drugs (ritonavir and efavirenz) may interfere and jeopardize the treatment process .
Efavirenz blocks the activity of the reverse transcriptase enzyme and the formation of DNA, without which HIV cells cannot divide and multiply. This drug does not cure HIV or AIDS, but it helps stop the infection of healthy cells in the body. Efavirenz is indicated for antiretroviral combination therapy for adults, adolescents, and children over 3 months old who are infected with HIV-1. Efavirenz is also prescribed to reduce the risk of HIV infection when combined with other antiretroviral therapy drugs as part of an extended prevention regimen for people at significant risk . The drug should not be used as an independent medicine since the resistance of the immunodeficiency virus to the active substance is quickly developed.
Ritonavir, the main component of Norvir, inhibits HIV replication, increases the number of CD4 cells in the blood and reduces the concentration of viral RNA, prolongs remission, improves hematological parameters. The drug is also used as a pharmacokinetic booster, which is based on the ritonavir inhibiting metabolism mediated by the cytochrome CYP3A4 isoenzyme . The degree of enhancement is related to the metabolism of the co-administered anti-HIV med and its effect on ritonavir metabolism. The maximum inhibition of metabolism with the combined use of an HIV protease inhibitor, as a rule, is achieved with a dose of ritonavir from 100 mg once per day to 200 mg twice per day and depends on the jointly used HIV protease inhibitor .
Sustiva and Norvir both contain active ingredients that are HIV inhibitors.
Coadministration of ritonavir with efavirenz may unpredictably affect the plasma concentrations of ritonavir. Specifically, ritonavir levels increase when it is given therapeutically as an antiretroviral agent and decrease when it is used at low dosages as a pharmacokinetic booster. In 11 healthy subjects given ritonavir, efavirenz increased the systemic exposure (AUC) and peak plasma concentration (Cmax) of ritonavir by 18% and 24% respectively. Efavirenz AUC and Cmax also increased by 21% and 14% respectively. The combination was associated with a higher frequency of adverse effects and laboratory abnormalities .