Cialis, ingredient tadalafil, relaxes muscles within the penis and allows increased blood flow into the penis, necessary to achieve and maintain an erection. Cialis is used to treat erectile dysfunction (impotence). It may also be used for purposes other than those listed in this medication guide. Taking Cialis with any of these medications may cause very low blood pressure, possibly resulting in dizziness, fainting, stroke, or heart attack. Cialis is usually taken when needed before sexual activity. The effects of this medicine may last for up to 36 hours or more. Your doctor will determine how often you can take this medicine.