Ritonavir

Ritonavir is an antiretroviral medication of the protease inhibitor class used in the treatment of HIV-1 infection. It was developed by Abbott Laboratories and marketed as Norvir; it was approved for medical use in the mid-1990s and remains a core component of many combination regimens.

In clinical practice, ritonavir is often given at low booster doses (for example 100 to 200 mg once daily) not primarily for its antiviral activity, but to inhibit the cytochrome P450 3A4 (CYP3A4) pathway. This strong, but selective, enzyme inhibition raises the blood levels of coadministered protease inhibitors such as lopinavir, darunavir, atazanavir, enhancing efficacy and allowing simpler dosing.

Ritonavir itself has antiviral activity but is usually not used as a sole therapy because the required high dose comes with excessive adverse effects. It is administered orally and has complex pharmacokinetics with variable bioavailability; it is extensively metabolized in the liver and eliminated mainly in feces.

Common adverse effects include nausea, diarrhea, vomiting, and fatigue. Longer-term use can contribute to metabolic complications such as dyslipidemia and insulin resistance; hepatotoxicity and pancreatitis have been reported rarely. The drug has extensive drug interaction potential due to CYP3A4 inhibition, requiring careful review of concomitant medications and, in many cases, dosage adjustments or avoidance.

In modern HIV therapy, ritonavir is frequently replaced or supplemented by alternative boosters such as cobicistat, or used within boosted protease inhibitor regimens. Its role is chiefly pharmacokinetic rather than direct antiviral activity in these contexts.