Cialis 20mg tablet belongs to a group of medicines called PDE5 (phosphodiesterase) inhibitors. It works by relaxing the blood vessels in your penis. This allows blood to flow into the penis and produce an erection when sexually aroused. This medicine will only help get an erection if you are sexually stimulated. It is very effective but needs to be taken at least 30 minutes before sexual activity.
Do not take this medicine if you also take medicines called nitrates (often given for chest pain).
Common side effects of Cialis include;

Flushing (sense of warmth in the face, ears, neck and trunk)
Headache
Blurred vision
Muscle pain
Upset stomach
Rash

Talk to your doctor if the side effects bother you or will not go away.
Dosage
Use this medicine in the dose and duration as advised by your doctor.
Swallow it as a whole. Do not chew, crush or break it. Cialis 20mg Tablet may be taken with or without food, but it is better to take it at a fixed time.