When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. If you have an erection that lasts more than 4 hours, get medical help right away. These medicines are called PDE5 inhibitors. Its effects might last longer if you have mild to moderate ED. Who might need a lower Viagra dose or extra monitoring? This boosts blood flow and helps you get an erection from sexual activity. You can take this medicine with or without food 30 minutes before sex, depending on the dose.It’s simple: When it works as intended, Viagra causes a man who is sexually stimulated to get an erection. How does Viagra do that? And why does Viagra work only if the . FAQ. What is Viagra? Viagra relaxes muscles found in the walls of blood vessels and increases blood flow to particular areas of the body. Viagra is used to treat erectile . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles .