While each PDE5 inhibitor differs slightly, they’re very similar in treating erectile dysfunction. We also looked at additional services, such as access to digital pharmacies and home delivery options. It works for up to 36 hours. 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. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. You can see an in-person provider or speak with a doctor virtually.Amazon Pharmacy: Sildenafil (Generic for Viagra, Oral Tablet).us. Account Returns& Orders. Cart. Transfer your prescription for Sildenafil (Generic for Viagra, Oral . Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is . Viagra® is available in three doses: 25 mg, 50 mg and mg. The 25 mg dose is the lowest available dosage of Viagra®, which is only used if you are unable to tolerate .