Sildenafil was first synthesized by pharmaceutical chemists working at Pfizer. It was initially studied for use in hypertension and angina. Phase I clinical trials suggested that the drug had little effect on angina but could induce penile erection. Sildenafil (Viagra®) was patented in 1996 and approved by the FDA for use in erectile dysfunction (ED) in March, 1998. Vardenafil (Levitra®) was approved by the FDA for treatment of erectile dysfunction in August, 2003 and tadalafil (Cialis®) in November, 2003. Avanafil (Stendra®) was approved in April, 2012. Udenafil (Zydena) is not FDA approved for sale in the United States, but is available in Korea, Russia and the Philippines.
