Levitra was the second impotency drug to be developed after Viagra and it would be difficult to discuss Levitra without first discussing Vigra. Viagra (Sildenafil Citrate) was created by a group of chemists working for Pfizer in England. The chemists were trying to create a medicine for the treatment  of 'Angina Pectoris' and Hypertension. During clinic trials researchers noted that although the drug didn't work in the treatment of 'Angina Pectoris', it did help  men gain penile erections. Pfizer decided to seek approval from the 'American Food and Drug Administration (FDA)' for the drug as a treatment for erectile dysfunction.

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Sexual arousal usually starts with some type of external stimuli which results in the activation of the Parasympathetic Nervous System (a division of the Autonomic Nervous System). The activation of the Parasympathetic Nervous System leads to the release of Nitric Oxide. The signaling molecule Nitric Oxide, which is derived from the amino acid L-arginine, enters the 'Corpus Cavernosum' of the penis and causes the levels of 'Cyclic guanosine monophosphate' (cGMP) to increase. cGMP regulates the conductance of ion channels and one of its effects is to relax the smooth muscle tissue in the Helicine arteries of the penis which results in vasodilatation and an in increase in blood flow to the penis with resulting erection. cGMP is broken down by the enzyme 'cGMP-specific phosphodiesterase type 5' which is known as PDE5 for short.

In men with erection problems, not enough blood is entering the penis. Viagra and other impotency drugs work in similar ways by stopping the PDE5 enzyme from breaking down cGMP which is the chemical that increases blood flow.

Viagra (Sildenafil Citrate) was patented in 1996 and gained FDA approval two years later. The drug went on sale in the United States in 1999 and initial annual sales are believed to have been over $1 billion dollars.

Due to the success of Viagra, two other medications for erectile dysfunction quickly followed. Levitra (Vardenafil) received FDA approval in August 2003 and Cialis (tadalafil) also received FDA approval in three months later in November 2003.