When Crixivan (indinavir, IDV) is co-administered with Norvir (ritonavir, RTV) 400/400 mg BID, IDV trough levels (lowest level before next dose), are substantially increased without affecting the total amount of IDV in the body over time, or the area under the curve (AUC). Also, the addition of RTV to IDV allows for twice-a-day dosing of IDV, without any food restrictions. Based on this knowledge, researchers analyzed the effect of adding RTV to IDV, in 35 people already on IDV (as their only protease inhibitor) with detectable viral loads. Seven people had to withdraw early from the study due to side effects, including gastrointestinal effects, anxiety, and blood lipid changes. IDV trough levels increased from 0.15 to 0.54 ug/mL, with no change in the AUC. In an on-treatment analysis, viral load was below 50 copies in 53% (9/17) at week 16, and was less than 400 copies in 59% (10/17) at week 16. Thus, in people with detectable, but low HIV RNA viral loads receiving an IDV-containing regimen, intensification with RTV increased IDV trough levels, and reduced viral load in a majority of people.