Cialis can be taken with or without food. If you are a Mayo Clinic patient, this could include protected health information. This increased blood flow can cause an erection. What special dietary instructions should I follow? After taking a single tablet, some of the active ingredient of Cialis remains in your body for more than 2 days.Healthcare providers may also prescribe tadalafil to manage two other health conditions involving insufficient blood flow: pulmonary arterial hypertension (PAH) . Cialis and generic tadalafil are available with a prescription only. There is no OTC version, though there have been efforts in recent years to develop a . Take tadalafil exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor. If you are taking tadalafil to treat PAH or BPH, follow your .