Your doctor may need to change the doses of your medications or monitor you carefully for side effects. How should this medicine be used? Sexual problems may be due to difficulties with arousal, a lack of desire or both. If you need emergency medical care for a heart problem, it is important that your doctor knows when you last took sildenafil. 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. If you become pregnant while taking sildenafil, call your doctor. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months.Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult .  · Viagra; Descriptions. Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines . 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 .