Cialis (tadalafil) is a phosphodiesterase inhibitor that increases the inflow of blood into the tissues of the penis allowing for a more stable and firm erection sufficient to complete sexual intercourse. It’s very well tolerated with such minor side effects as muscle and back pain, headache, facial flushing, indigestion and diarrhea. If your symptoms change in intensity or prevent you from performing your everyday tasks talk to your health care provider as you may be required a dose adjustment. Also make sure you mention to your doctor any health conditions that can interfere with your treatment.