Cobicistat/TYBOST is a booster drug used only in the combination pills, EVOTAZ, GENVOYA, STRIBILD, and PREZCOBIX.  Cobicistat works by inhibiting certain specific liver enzymes, so that some drugs can remain active for longer in your system. Cobicistat has no anti-HIV properties on its own.  It is generally well tolerated but could make already existing kidney/liver problems worse so regular kidney/liver function tests should be done as a precaution.