Its effects often last for 4 to 5 hours. There are online sources that will provide “Viagra” without a prescription, but this is an illegal action and, in the interests of your own safety, you should only consider purchasing Viagra from reputable and legal sources. Along with other basic personal information such as your birth date and social security number , remember to have your health insurance ID number handy for when you log-in online or speak to a representative on the phone. If Viagra is covered by your insurance, the prescription co-pay is usually the same at all pharmacies, so there is no need to shop around for the best price. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. · Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Stop taking sildenafil and . CHOOSE BRAND-NAME VIAGRA. The original little blue pill prescribed by doctors for over 20 years. Clinically proven to treat men with erectile dysfunction (ED) Ongoing . Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects often last for 4 to 5 hours. It might work longer if you have mild to moderate erectile dysfunction. Vardenafil. See more.