Phosphodiesterase inhibitors: phosphodiesterase inhibitors which are dependent on CYP3A4 metabolism, such as tadalafil and sildenafil, are expected to result in an approximately 2-fold and 11-fold increase in AUC respectively, when co-administered with ritonavir containing regimens including Kaletra and may result in an increase in PDE5 inhibitor associated adverse reactions including hypotension, synope, visual changes and prolonged erection.