Never take Generic Cialis (tadalafil) if you are currently using any of the following drugs (no matter if you use them on a regular basis or only occasionally):
nitroglycerin-type medications used to treat heart problems or chest pain (amyl nitrite, isosorbide dinitrate, isosorbide mononitrate, nitroglycerin, etc).

It’s also known that under certain circumstances Cialis (tadalafil) may interact with the following medications and substances:
alpha blockers (meds used in the treatment of high blood pressure and benign prostatic hyperplasia), including

• alfuzosin (UroXatral®)
• doxazosin (Cardura®)
• prazosin (Minipress®)
• or terazosin (Hytrin®);
• bosentan

a number of certain antibiotics, including clarithromycin, erythromycin, troleandomycin;
a number of certain drugs used in the treatment of seizures, including

• carbamazepine
• phenytoin, and phenobarbital
• cimetidine
• cisapride
• diltiazem
• grapefruit and grapefruit juice

medicines used to fight fungal infections (fluconazole, itraconazole, ketoconazole, voriconazole); mibefradil
nicardipine certain medications used in the treatment of HIV and AIDS quinidine rifabutin
rifampin or rifapentine certain pills used in the treatment of depression, anxiety or other mood problems (such as fluoxetine, fluvoxamine, nefazodone), verapamil.