Cialis is a phosphodiesterase inhibitor that relaxes certain muscles in the penis providing for a better blood inflow. The medicine needs to be taken prior to sexual activity and some stimulation is required to get an erection. Cialis should be taken only once a day. Do not exceed the dose prescribed by your doctor or take a double dose, as it will not improve your sexual performance if compared with one dose of the drug, and may lead to serious side effects. A rare complication is possible in patients taking Cialis - an erection that lasts over 4 hours and is painful (priapism). This condition may permanently damage your penis, so make sure you seek immediate medical help in case of having an erection for longer than 4 hours.
If you are also using any nitrate drugs for heart problems you mustn't take Cialis simultaneously, as dangerous side effects are possible (sudden drop of blood pressure, leading to a stroke or a heart attack). If you experience any of the following symptoms stop taking the drug and seek immediate medical help: tingling or numbness in your chest, neck, jaw or arms, dizziness or nausea during sexual activity.