Cialis is classified as a phosphodiesterase-5, or PDE5, inhibitor. This means that it delays the action phosphodiesterase type-5 enzyme. This particular enzyme is works in several areas of the body, including the penis, to relax smooth muscle tissue. If the muscles relax, the penis does not receive adequate blood flow to achieve or maintain an erection. PDE5 inhibitors block the enzyme's action, allowing the penis to retain the needed blood supply.

Cialis - manufactured by Lilly, is available as a low-dose tablet for daily use or as a single dose the patient uses on an as-needed basis no more frequently than once daily. The daily use dosage is a 2.5 or 5 milligram tablet taken every day at the same time regardless of when sexual activity will occur. Standard dosage for use as needed is a 10mg tablet, which should be taken approximately 30 minutes prior to sexual activity, but the tablets are also available in 5mg and 20mg strengths.

Side effects may range from mildly uncomfortable to life threatening. Patients should consult with their doctor immediately should they experience chest pain, a heaviness or tightness in the chest or an irregular or racing heartbeat. Pain in the arm, jaw or back, blurred or double vision, severe headaches or vomiting are symptoms that should be promptly reported to the doctor. Fainting, confusion, dizziness, hearing problems and breathing difficulties need to be reported. Patients should seek medical attention if an erection is painful or lasts longer than four hours. The doctor needs to be notified if the patient develops a rash, hives or other symptoms of an allergic reaction. Seek medical attention if the patient cannot speak or has difficulties with his speech.