Urological Health Penis Health Erectile Dysfunction How Viagra Works By Mark Cichocki, RN twitter linkedin Mark Cichocki, RN, is an HIV/AIDS nurse educator at the University of Michigan Health System for more than 20 years. Learn about our editorial process Mark Cichocki, RN Medically reviewed by Medically reviewed by Chris Vincent, MD on November 09, 2019 linkedin Chris Vincent, MD, is a licensed physician, surgeon, and board-certified doctor of family medicine. Learn about our Medical Review Board Chris Vincent, MD on November 09, 2019 Print Thomas Tolstrup / Digital Vision / Getty Images Mature men are discovering a newfound sexual youth because of the little blue pill called Viagra — which is used to treat erectile dysfunction (ED), or impotence, as it's often called. Let's learn more about Viagra, how it's dosed, and what to watch out for when a person has been prescribed this medication. Are the Causes of Erectile Dysfunction Avoidable? How It Works Viagra (sildenafil) belongs to a group of medicines that delay the enzymes called phosphodiesterase from working too quickly. By controlling phosphodiesterase, sildenafil helps to maintain an erection that is produced when the penis is physically stimulated. Prescription Guidelines Viagra comes in tablet form in dosages of 25mg, 50mg, and 100mg. Men Up to 65 Years of Age: Viagra 50mg by mouth is usually started. It's prescribed to be taken once a day, 30 minutes to 4 hours before intercourse. In situations where 50mg isn't effective, a person's doctor may prescribe 100mg to be taken once daily 30 minutes to 4 hours before intercourse. For Men Older than 65 Years of Age: Often a lower dose is started, like 25mg. It's taken 30 minutes to 4 hours before sexual intercourse. In some circumstances, men older than 65 may have their doses increased. Important Caution: Under no circumstances should men increase their Viagra doses without first consulting their physician. It's also important to be aware of interactions with heart or high blood pressure medications, which can be dangerous. Viagra and other phosphodiesterase medications — such as Levitra (vardenafil), Cialis (tadalafil), and Stendra (avanafil) — should never be taken with any form of organic nitrates or nitrites (e.g., nitroglycerine and amyl nitrite, used for angina), nor should they be taken along with guanylate cyclase stimulators (e.g., riociguat, used to treat high blood pressure in the lungs). Men also should not take Viagra if they are also taking any other medication for high blood pressure, such as Revatio (a form of sildenafil) or other drugs. Because phosphodiesterase medications such as Viagra may lower blood pressure, they should be used cautiously in men with heart disease (e.g., angina, history of heart attack, stroke, or narrowing of the aorta). When in doubt, discuss with your doctor if it is safe for you to take Viagra. Potential Side Effects Headache Heartburn Diarrhea Feeling of warmth Nosebleeds Nasal congestion Sleep disturbance Numbness, tingling, or burning in extremities Muscle aches Changes in color vision Sensitivity to light Serious side effects that require a person to call their doctor right away include blurry vision, loss of vision, fainting, dizziness, chest pain, difficulty breathing, difficulty hearing, ringing in ears, rash, pain when urinating, painful erection, or an erection that lasts more than 4 hours. Precautions If you think you need Viagra, please see your doctor and inquire about a workup and a prescription for your erectile dysfunction. Do not order Viagra online — the FDA warns about the potential harm of obtaining Viagra-like products through the Internet, as their safety has not been studied, and their ingredients may be harmful to the consumer. Also, the use of Viagra may be associated with higher levels of unprotected anal sex with an HIV-positive partner, according to a 2002 study in AIDS. Practicing safe sex, regardless of whether you use Viagra, is critical for your own sexual health and the health of your partner. Erectile Dysfunction Doctor Discussion Guide Get our printable guide for your next doctor's appointment to help you ask the right questions. Download PDF Email the Guide Send to yourself or a loved one. Sign Up This Doctor Discussion Guide has been sent to {{form.email}}. There was an error. Please try again. Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Viagra prescription drug information. Pfizer. December 2017. Additional Reading FDA. (2009). Hidden Risks of Erectile Dysfunction "Treatments" Sold Online. Kim AA, Kent CK, & Klausner JD. Increased risk of HIV and sexually transmitted disease transmission among gay or bisexual men who use Viagra, San Francisco 2000-2001. AIDS. 2002 Jul 5;16(10):1425-8. U.S. National Library of Medicine. (2015). Sildenafil.