Viagra is a prescription medication used to treat erectile dysfunction (ED) in men. The active ingredient in Viagra is sildenafil citrate, which works by relaxing the muscles in the walls of blood vessels, increasing blood flow to the penis and helping men achieve and maintain an erection. 
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Viagra is generally taken 30 minutes to an hour before sexual activity and is effective for up to four hours. It should not be taken more than once a day, and it is important to follow your doctor's instructions when taking Viagra.

Like all medications, Viagra can have side effects, although not everyone experiences them. Common side effects of Viagra may include headache, facial flushing, indigestion, and nasal congestion. Rare but serious side effects may include sudden vision loss, hearing loss, or an allergic reaction. If you experience any of these serious side effects, you should seek immediate medical attention.

It is important to note that Viagra should not be taken by individuals who are taking medications that contain nitrates, or by those with certain medical conditions such as severe heart or liver problems. It is also important to discuss any medications you are taking and any medical conditions you have with your doctor before taking Viagra.