You never know when your urge to have sex would be there. And if you take erectile dysfunction pills you must have it before the effects of the pill disappear so that you enjoy having sex. While other ED (Erectile Dysfunction) pills have effects for around 4 to 5 hours Cialis is the only ED pill which claims to have effect for 24 hours to 36 hours. That means you have no need to worry about the timing of having sex. You can enjoy it anytime you like. The only difference between Levitra and Cialis is the above stated aspect of duration of the effectiveness of the drug onto your blood flow. It works by relaxing your penis muscles which in turn leads to flow of blood through the artery. And as the blood flows in the penis gets stiffer and you get an erection. At that juncture, the swelling of the penis blocks the flow of the blood from the penis through the vein and this in turn keeps the penis erect. And this process is maintained till the time the penis ejaculates. Thus, it means that Cialis just helps in stiffening penis by relaxing penile muscles and increasing the blood flow to it. As the marketers of it Lilly ICOS LLC says, it should not be taken by women and children, the advice should be followed. A person with heart problems, diabetes, blood pressure or allergy should consult a doctor before taking the pill. Do not take the pill more than prescribed. The common side effects are headache, muscle pain, stomach upset and back pain. Other minor side effects of Cialis include flushing and nasal congestion. In case of side effects it is recommended to visit your doctor. Last but not the least, the point to be noted is that, Cialis which is an Alternative of Levitra does not differ from Levitra by much but by the duration of the effectiveness of the pill.