Viagra - http://www.viagra.com/ Pfizer's official site for this drug, with information for both patients and medical professionals.
Sildenafil Citrate - http://www.rxlist.com/cgi/generic/viagra.htm Information from the RxList.
Viagra Consumer Information - http://www.fda.gov/cder/drug/InfoSheets/patient/sildenafilPIS.htm FDA consumer information page about the drug, with updates on adverse effects and interactions.
FAQ About Drugs to Treat Erectile Dysfunction (Impotence) - http://www.intelihealth.com/IH/ihtIH/EMIHC000/9105/28955/213545.html?d=dmtContent Information on how they work, safety, efficacy and side effects. Reviewed by Harvard Medical School.