Viagra, Silagra, Cialis, Kamagra, Kamagra Jelly... are drugs used for treating erectile dysfunction or ED. They have also become known as performance enhancing agents through research. Though they do not significantly affect perfromance at zero altitude or sea level, they are supposed to improve maximum exercise capacity due to relived constriction of blood vessel in higher altitudes. All above mentioned ED drugs are very similar in action, but they do differ in time needed to produce desired effect ( erection ) and the time the erection is sustained. Viagra was the first of this revolutionary group of medications and Cialis is the most potent and long lasting (up to 36 hours) that is why it is dubbed the weekend pill.