The bottom line: It’s safe to take as prescribed. Medicines that block an enzyme called CYP3A4. If you don’t have a prescription, the drug itself — not just how you use it — may be a problem. In psychogenic erectile dysfunction, psychological issues prevent maintaining or getting an erection — nothing physical. Its effects might last longer if you have mild to moderate ED. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction.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 . If you’re looking for a less costly Viagra, you might want to ask your doctor about sildenafil. It’s the active ingredient in both branded Viagra and the drug Revatio, . 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 .