While this may be the case, researchers also know that when two or more PIs are combined, the effects each drug may have on the blood level of other drugs could be different. For example, researchers know from some recent studies that the combination of Norvir, Lexiva, and Kaletra, another PI, leads to an unacceptably low level of both Kaletra and Lexiva. Because researchers can not always assume that when multiple HIV medications are combined, the levels will remain high enough to be effective, the investigators think it will always be reasonable that, before any combination of drugs are used on HIV-infected patients, the effect a combination has on the levels of each of the drugs in the combination should be investigated.