Thanks for the question. The intake of sildenafil can prolong the effect of cardizem. The intake if these drugs causes only moderate interaction, but it is still better to watch out for some symptoms. These drugs taken together or any one can cause lowering of BP. Some of the other symptoms to watch out are chest tightness, shortness of breath, visual disturbances, fainting and prolonged erection(about 4 hours). You can consider some of the other drugs like Counadin, Benicar, Aggrenox and Viagra are known for serious drug interaction. I hope this helps.