Viagra is a prescription medicine used to treat erectile dysfunction (ED). It contains sildenafil as the active ingredient. Viagra works by relaxing blood vessels in the penis, increasing blood flow and helping you achieve and maintain an erection when sexually aroused.

Viagra is highly effective for most men. Around 70–80% of men are able to achieve and maintain an erection firm enough for sex after taking it. It starts working in about 30 to 60 minutes, and its effects can last for up to 4 hours. For best results, take it on an empty stomach and only when you plan to be sexually active.

The effects of Viagra usually last for around 4–6 hours. During this time, you may get an erection with sexual stimulation. It does not cause constant erections and only works when you are aroused.

Take Viagra with a glass of water about 30–60 minutes before planned sexual activity. Do not take more than one dose in 24 hours. It can be taken with or without food, but heavy or high-fat meals may delay the effect. Avoid excessive alcohol as it can reduce effectiveness and increase side effects.

Viagra comes in 25mg, 50mg and 100mg strengths. The usual starting dose is 50mg. Depending on your response and any side effects, your prescriber may adjust this down to 25mg or up to 100mg.

Common side effects include headache, flushing, dizziness, indigestion and nasal congestion. Some men may also experience blurred vision or changes in colour perception. Rare but serious risks include chest pain, sudden vision or hearing loss, and a prolonged painful erection lasting more than 4 hours (priapism). If these occur, seek medical help immediately.

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There is no difference in how they work. Sildenafil is the generic name for the active ingredient, while Viagra is the original branded version. Both are medically identical. The main difference is price, as generic sildenafil is usually cheaper than branded Viagra.