Viagra is a drug produced by Pfizer for the treatment of male erectile dysfunction ( ED ), often called impotence.

It can help many men to get and keep an erection when they become sexually stimulated. It comes in pill form and is taken before sexual activity.

Some degree of ED is common in about 50% of men aged 40 to 70 years.

Viagra works by increasing blood flow to the penis. When a man is sexually stimulated, the arteries in the penis relax and widen, letting more blood flow in. As the arteries expand, the veins that normally carry blood away from the penis become compressed, restricting blood flow out. With more blood flowing in and less out, the penis enlarges, resulting in an erection.

Pfizer claim that Viagra improves erections in up to 80% of men who take it, regardless of how long they have had ED, or how old they are.

It is taken about an hour before sexual activity and will not cause an erection unless the man is sexually aroused either physically or visually.

Initial starting dose is 25mg and it should not be taken more than once a day.

Viagra should not be taken by men who suffer from heart disease or those using nitrate drugs, as the sudden drop in blood pressure could result in a heart attack or a stroke.

The side effects include headache, facial flushing, stomach upset and blurred vision.

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"Although now in our seventies, with the aid of painkillers, my wife and I make love every Sunday night. In view of the AIDS scare, do you think we should use a condom?"