Sildenafil Viagra. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. Blood flows into two erection chambers in the penis, which are made of spongy muscle tissue called the corpus cavernosum CC. After orgasm, the muscles of the penis relax, releasing blood back into the circulation system. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects.Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription . The U.S. Food and Drug Administration has approved the use of sildenafil (brand name Viagra) only in “men” older than 18 years. Pfizer, the company that manufactures this . At age 40, 5% of males are diagnosed with complete ED, but this number increases to 15% by age In this article, we’ll discuss why ED becomes more common with age as well .