A clinical study utilising a cocktail of drugs that are metabolised by cytochromes CYP2C9, CYP2C19 and CYP2D6 demonstrated an increase in CYP2C9 and CYP2C19 activity and inhibition of CYP2D6 activity in the presence of PREZISTA/rtv, which may be attributed to the presence of low dose ritonavir.