Viagra is a brand name for the drug sildenafil, which belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors. Viagra is used to treat erectile dysfunction (ED), which is the inability to get or keep an erection hard enough for sexual activity. Viagra works by increasing blood flow to the penis, but only when you are sexually aroused. You should take Viagra as prescribed by your doctor. The usual dose is 50 mg, taken about an hour before sexual activity. However, you can take it anywhere from 30 minutes to 4 hours before sexual activity. Do not take more than one dose per day. You can take Viagra with or without food, but avoid high-fat meals as they may delay the onset of action. The effects of Viagra vary from person to person, depending on factors such as age, health, metabolism, and other medications. Generally, Viagra lasts for about 4 to 6 hours, but it may last longer or shorter for some people. It is important to note that Viagra does not make you sexually aroused or increase your sexual desire. You still need sexual stimulation for Viagra to work. Viagra is generally safe and well tolerated, but like any medication, it can cause side effects. The most common side effects are headache, flushing, indigestion, nasal congestion, dizziness, nausea, hot flushes, and vision changes (such as blurred vision, blue-tinted vision, or increased sensitivity to light). These side effects are usually mild and temporary, and they should go away within a few hours. However, some people may experience serious side effects from taking Viagra. These include: an erection that lasts longer than 4 hours (priapism), sudden vision loss in one or both eyes, sudden hearing loss or ringing in the ears (tinnitus). These are rare but serious conditions that require immediate medical attention. You should not take Viagra if you are allergic to sildenafil or any of the ingredients in Viagra, if you are taking nitrates or nitric oxide donors (such as amyl nitrite) for chest pain or heart problems, if you are taking riociguat (Adempas) for pulmonary arterial hypertension (PAH), if you have severe heart or liver problems, if you have low blood pressure or high blood pressure that is not controlled, if you have had a stroke or a heart attack in the last 6 months, if you have a rare inherited eye disease called retinitis pigmentosa, or if you have ever had vision loss due to NAION. You should also tell your doctor about all the medicines you are taking or plan to take, including prescription and over-the-counter medicines, vitamins, and herbal supplements. Some of the medications that can interact with Viagra are: alpha-blockers (such as doxazosin or tamsulosin) for high blood pressure or prostate problems, protease inhibitors (such as ritonavir or saquinavir) for HIV infection, antibiotics (such as clarithromycin or erythromycin) or antifungals (such as ketoconazole or itraconazole), other PDE5 inhibitors (such as tadalafil or vardenafil) for ED. These medicines can affect the level of Viagra in your blood and increase the risk of side effects. You should also limit or avoid alcohol consumption when taking Viagra, as alcohol can reduce its effectiveness and increase the risk of side effects. Alcohol can also worsen some of the underlying conditions that cause ED, such as diabetes, heart disease, or high blood pressure. There is a generic version of Viagra called sildenafil. Sildenafil contains the same active ingredient as Viagra and works in the same way. However, it may have different inactive ingredients, such as fillers, colors, or flavors. Sildenafil is usually cheaper than Viagra, but it may not be available in all countries or pharmacies. There are also many products that claim to be herbal or natural alternatives to Viagra, but they are not proven to be safe or effective. Some of these products may contain unknown or harmful ingredients that can cause serious health problems. Some of these products may also interact with other medications or medical conditions. Therefore, it is best to avoid these products and consult your doctor before trying any alternative treatments for ED.