Ritonavir is a Protease Inhibitor (PI) but is usually used to enhance the performance of other drugs it was approved in 1996 and again in 2000.  Take ritonavir with food. it is generally well tolerated with some mild side effects including stomach problems and general malaise in the first few weeks.  Store ritonavir in the refrigerator or in a cool place.  It is often combined into a single pill with lopinavir.