This increased blood flow can cause an erection. You can take it in a small dose daily or in a larger dose as needed. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. These are commonly prescribed for an enlarged prostate or male pattern baldness. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. If you become pregnant while taking sildenafil, call your doctor.You should not use Viagra if you are allergic to sildenafil, or: if you take other medicines to treat pulmonary arterial hypertension, such as riociguat (Adempas). if you take nitrates. . No, you should not take Viagra if you don’t have ED. Viagra is a prescription medication that can have serious side effects. And it’s approved only to treat ED, not to be used . Viagra or generic sildenafil for ED should not be used in women. For seniors: The kidneys, liver, or heart of older adults may not work as well as they used to. This can cause your .