When individuals who are infected with HIV are started on treatment with HIV medications, the effect of these drugs only lasts for a limited period of time, often because of development of drug resistance by the HIV virus. When this happens, such patients have to be switched to different combinations of HIV medications. However, since the availability of new HIV drugs that are active against resistant virus is limited, HIV care providers are resorting to curtail medications that contain three or more protease inhibitors (PIs). The reason for this is Norvir (ritonavir), a PI that has the ability to boost or increase the blood levels of other PIs in a way that can sometimes overcome the resistance of HIV virus. In addition, it may be more difficult for the virus to overcome two or more drugs with high blood levels, than it is to overcome just one. For these reasons, many clinicians are now using Norvir in combination with two other PIs, including Crixivan (indinavir) plus Lexiva (fosamprenavir), for treating patients who have been exposed to many other HIV medications.