This site is intended only for U. The amount of medicine that you take depends on the strength of the medicine. 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. What special precautions should I follow? Continue to take sildenafil even if you feel well. If your dose is different, do not change it unless your doctor tells you to do so.If you’re interested in buying Viagra online, five of the best options available are Hims, Roman, Lemonaid, Optum Store, and REX MD. Read on for full details on the process . If you experience chest pain, dizziness, or nausea during sex, seek immediate medical help. VIAGRA can cause serious side effects. Rarely reported side effects include: an erection that will not go away (priapism). sudden vision loss in one or both eyes. sudden hearing decrease or hearing loss. Key facts. Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work.