| Bosentan | CYP3A4 inducer | Sildenafil | Sildenafil levels fall 50%; bosentan levels increase 50% |
| CYP3A4 substrate | Ciclosporin | Ciclosporin levels fall 50%; bosentan levels increase 4-fold. Combination contraindicated |
| CYP3A4 substrate | Erythromycin | Bosentan levels increased |
| CYP3A4 substrate | Ketoconazole, itraconazole | Bosentan levels increased |
| CYP2C9 inducer | HMG CoA reductase inhibitors | Simvastatin levels reduced 50%; similar effects likely with atorvastatin |
| CYP2C9 inducer | Warfarin | Increases warfarin metabolism, may need to adjust warfarin dose |
| CYP2C9 and CYP3A4 inducers | Hormonal contraceptives | Hormone levels decreased, contraception unreliable |
| Sitaxentan | CYP2C9 inhibitor | Warfarin | Inhibits warfarin metabolism, warfarin dose needs to be reduced |
| ? inhibition of OATP transporter | Ciclosporin | Increases sitaxentan levels 6-fold; combination contraindicated |
| Sildenafil | CYP3A4 substrate | Bosentan | Sildenafil levels fall 50%; bosentan levels increase 50% |
| CYP3A4 substrate | HMG CoA reductase inhibitors | May increase simvastatin/atorvastatin levels through competition for metabolism |
| CYP3A4 substrate | HIV protease inhibitors | Ritonavir and saquinovir increase sildenafil levels |
| CYP3A4 substrate | Erythromycin | Sildenafil levels increased |
| CYP3A4 substrate | Ketoconazole | Sildenafil levels increased |
| CYP3A4 substrate | Cimetidine | Sildenafil levels increased |
| cGMP | Nitrates, nicorandil | Profound systemic hypotension |