The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Continue to take sildenafil even if you feel well. You can take it up to 4 hours before you want to have sex. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine.VIAGRA can cause your blood pressure to drop suddenly to an unsafe level if taken with certain other medicines. Do not take VIAGRA if you take any other medicines called “nitrates.” Nitrates are used to treat chest pain (angina). A sudden drop in blood pressure can cause you to feel dizzy, faint, or have a heart attack or stroke. What Happened When I Took Viagra Recreationally. 1. Viagra was effective for much longer than four hours. Shutterstock. Viagra comes in three different sizes: 25mg, 50mg, or mg. In the past, I’ve found that 25mg always did the job just fine. However, my journey begins with a bottle of the mg tabs which I’ve chosen to cut into thirds. Brand names for sildenafil include Viagra, Revatio, and Liqrev. The cost for sildenafil oral tablet mg is around $10 for a supply of 2 tablets, depending on the pharmacy you visit. CVS Pharmacy, Target, WalMart Pharmacy, Duane Reade and 65, pharmacies nationwide. No manufacturer promotions could be found for this medication. Patient.