The mechanism of action for ED pills is simple: They inhibit the PDE5 enzyme which regulates the signals responsible for the smooth muscle tissue. Erectile dysfunction can be caused by a variety of factors , including age, cardiovascular problems, diabetes, and other conditions that affect nerve function or blood flow. Oral medicines for ED usually work well and cause few side effects. Popular ED medications found online include: Viagra sildenafil : As the first FDA-approved PDE5 inhibitor for ED, sildenafil is taken 30 minutes to four hours before sexual intercourse to help achieve and maintain an erection. Erectile dysfunction is the persistent inability to sustain or maintain an erection. Cialis tadalafil : This ED medication works with the same principle as Viagra, but lasts up to 36 hours. This information is reviewed by a licensed practitioner who will assess your eligibility for the medication.Online Pharmacy & OTC. Our 12 Picks for Online Pharmacies of If you need medication delivered here are the best online pharmacies—and how to choose the right . Still, if your doctor says it’s okay for you to use and you end up paying out-of-pocket, you can still get the generic for a lot less than Viagra. According to price-comparison site . We offer almost every brand of erectile dysfunction medication offered in the United States, including Viagra, Cialis, and Levitra. If you would like a refill of your current prescription .