What is Viagra?
Viagra (also known as sildenafil) is the first oral medical treatment that can prompt and enhance erections in men. Because the medicine is taken as a tablet and produces erections only in response to sexual stimulation, it has been heralded as one of the greatest developments in the treatment of male erectile dysfunction (impotence).

The history of Viagra
Viagra was to a large extent discovered by chance. The medicinal compound sildenafil citrate was initially developed to treat patients who had problems with their blood, or heart disease. During these early clinical trials, an unexpected 'side effect' was discovered - that the medicine could improve and maintain a man's erection.

Clinical trials of this effect were started in healthy male volunteers and then in patients with male erectile dysfunction. These clinical trials went faster than most other drug trials, because men were enthusiastic to try the compound and relatively few dropped out during the study period. Since then, the number of peer-reviewed medical publications (studies that are published after scrutiny and comment by experts in the field) has grown enormously. In 1997, there were less than 10 medical publications on the research database MEDLINE. In 1998, there were over 120 publications and, in 1999, over 200 reports.

What is male erectile dysfunction?
Male erectile dysfunction, or impotence as it was previously known, is the inability to maintain an erection that is sufficiently hard or that fails to last long enough for satisfactory sexual intercourse for both partners. It is important to know that premature ejaculation is not necessarily a sign of erectile dysfunction. Low sex drive (poor libido) can be associated with erectile dysfunction, particularly if there is an underlying hormonal disorder, but in the majority of cases, the patient seeks advice because libido is maintained while performance is not.

It is a common problem that men suffer, causing progressive difficulty with increasing age. Overall, approximately 1 man in 10 is affected by ongoing erectile dysfunction. Many early estimates of the frequency of erectile dysfunction came from the Massachusetts Male Ageing Study, which probably underestimated the frequency of problems. A more recent study of 1661 men in the Netherlands found much higher rates with up to 20 per cent of men between 50 and 54 years and 50 per cent of men between 70 and 74 years having some problems with erections.

What causes erectile dysfunction?
An erection is a complex process. To achieve and maintain a good erection, four parts of the body need to function well.

The hormonal environment needs to be right.

Several systems of nerves need to interact effectively.

Two systems of blood vessels (arteries and veins) must function well.

The man's state of mind, which overlays and influences the process.

An erection occurs in stages.

To start an erection, the brain sends messages that control hormone levels, the nerves and the blood vessels. If any factors interfere with this messaging system, one of these parts of the system will fail and a poor quality erection will result.

The first visible part of an erection is called the tumescent stage. During this stage, the cavernosal bodies (corpora cavernosa) in the shaft of the penis start to fill with blood and the penis becomes swollen but is not usually hard enough for sex.

As these cavernosal bodies fill up with blood, they squash the veins within the penis against the fascia (membranes) inside the penis. Blood is trapped within these blood vessels, which makes the penis hard and erect.

Erectile dysfunction can have either a physical or a psychological cause. Most commonly, both causes contribute to difficulties. Psychological causes include stress, depression or any form of anxiety. Erectile difficulties can in themselves cause a psychological state ('performance anxiety') that increasingly interferes with the erectile process. Although psychological factors often contribute to the problem in all age groups, physical causes become more common with increasing age. As a general rule, erectile dysfunction due to psychological causes has a more sudden onset than that caused purely by physical factors, which develops more progressively. Common physical causes include:

diabetes (which can result in erectile dysfunction in men as young as the early 40s)

narrowed or blocked arteries

hormone deficiencies

certain medicines

neurological disorders, such as multiple sclerosis or spinal cord injuries

previous abdominal and pelvic surgery, including prostate operations.

The proportion of patients with erectile dysfunction varies between different diseases. Erectile dysfunction is reported in up to 40 per cent of patients with narrowing of the arteries; in up to 52 per cent with hypertension (high blood pressure); in 61 per cent with ischaemic heart disease; in up to 64 per cent with diabetes; and in up to 90 per cent with severe depression.

How is erectile dysfunction treated?
Most patients with erectile dysfunction can be treated by one means or another. The choice of therapy depends on the underlying cause, and includes:

treatment of specific psychological causes, such as depression

psychosexual counselling

hormone treatment

treatment with medicines

vacuum devices

surgery.

Medical treatments work by:

either boosting the appropriate hormones

or directly stimulating the blood vessels involved (vasoactive compounds)

or promoting and prolonging the erectile mechanism once it has started.

Until recently, these medicines had to be used either as an injection into the side of the penis or as a pellet inserted into the urethra (urine tube in the penis).

Over the past few years, many attempts have been made to produce successful oral medications. Some of these have been successful, but the medicines have usually had significant side effects. Viagra is therefore a substantial improvement because it can be used safely and effectively in the vast majority of patients.

How does Viagra work?
Viagra promotes erections by relaxing the smooth muscle of the blood vessels thus increasing blood flow in the penis in response to sexual stimulation. It does this by specifically blocking a particular enzyme (protein that assists chemical reactions) called phosphodiesterase type 5 (PDE 5). This is the enzyme that normally breaks down chemicals causing the erectile response. Therefore, by blocking the breakdown of erectile chemicals, the drug promotes a harder and more prolonged erection. Viagra promotes penile erection only in response to sexual stimulation and has no effect in the absence of sexual stimulation.

Therefore, if a healthy man took Viagra and occupied himself with usual daily activities, he would not be expected to develop an erection within the five or six hours in which the drug is active unless sexual stimulation took place.

How well does Viagra work?
Many studies have compared Viagra to other therapies in men with erectile dysfunction from many causes.

Viagra improves erections in 78 per cent of men with erectile dysfunction regardless of the cause.

Specifically, it can improve erections in 70 per cent of patients treated for hypertension and in 57 per cent of patients with diabetes.

Single doses of Viagra enable more men to achieve an erection and to maintain this erection for longer than with placebo (dummy tablets).

Single doses of Viagra seem to produce dose-related rigidity at the base and the tip of the penis in response to looking at sexually stimulating images.

Viagra has also been linked with an increased rate of erections.

However, Viagra does not stimulate sexual desire and is not an aphrodisiac.

What about women?
Undoubtedly, Viagra is very effective in male erectile disorders, so some suggest that it might also be beneficial in women with sexual problems. Results from some early studies looked promising, particularly in women with antidepressant-induced sexual dysfunction. However, more recent trials have been less successful in women with female sexual arousal disorder, reduced sexual desire and dyspareunia (painful sex). Treatment with Viagra was well tolerated in women and the results from further studies are awaited.

What are the side effects and cautions?
Erectile dysfunction and cardiovascular diseases have common risk factors, therefore many of the patients who might benefit from Viagra also have cardiovascular diseases. A recent Californian review of the evidence concluded that Viagra was highly effective and overall the frequency of cardiovascular side effects was low and similar to that seen with placebo. However, it must not be used in patients who need to take nitrate medications and should be used with care in patients who get angina (heart pain) on exercise. Viagra can adversely interact with some medications, including those used to treat HIV.

More common side effects that all patients should be warned about are the possibility of headaches, indigestion due to acid regurgitation and very mild effects on vision. These visual changes make bright lights appear even brighter. Viagra is contraindicated (should never be used) in one very rare eye disorder called retinitis pigmentosa.

You can get more detailed information about the cautions, contraindications and side effects associated with Viagra in our medicines section.

How can I get Viagra?
Perhaps the greatest stumbling block for Viagra is its potential cost. It is significantly cheaper than many of its alternatives such as penile injections. However, because its tablet formulation makes it much more popular with patients, its market and thus the overall cost to the health service is potentially enormous. This threat has led the Department of Health to issue very strict prescribing guidelines. Therefore, some patients obtain Viagra on a private prescription, which means they have to pay for the medicine and usually the costs of prescribing.

Recent analyses of medical economics have suggested that Viagra or vacuum devices should be first-line therapy, with penile injection therapy kept as second-line therapy. Experts have pointed out, however, that a cascade of different therapies is the most costly option and therefore the doctor should involve the patient early in treatment choices and aim to achieve long-term patient satisfaction. The Internet has already greatly affected access to some medicines. Viagra can be obtained over the Internet, but usually at a higher cost. Also, online prescriptions, which are an outgrowth of the mail-order pharmaceutical business, might be convenient but can present several legal, ethical and safety issues.

Future options
Within the next few years, several alternative oral medications will be available in the UK for certain patients with erectile dysfunction. In development are other PDE 5 inhibitors that are similar to Viagra but possibly with fewer side effects, as well as medicines that act more on the brain, either those which relax smooth muscle (alpha-adrenergic antagonists) or morphine-related medicines, such as apomorphine.

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