It is the little blue pill, made in Britain, that revolutionised sex lives the world over.
Launched in 1998, this year Viagra turns 20 – and the best-known drug treatment for erectile dysfunction, prescribed to more than three million men in the UK, is going to be more easily available than ever.
From this spring, men across the UK will be able to buy it from a pharmacist without needing a prescription, providing they meet stringent medical criteria, reports The Daily Mail.
But despite being one of the most talked-about drugs of the 21st Century many myths and misconceptions still surround the treatment and the subject of erectile dysfunction.
Here, leading experts in the field reveal everything you never knew about Viagra and the things you really ought to…
'I TAKE IT EVERY DAY, IT'S LIKE POPPING AN ASPIRIN'
Professor Roger Kirby, consultant urologist and director of The Prostate Centre in London, is a leading light in the field, having published more than 300 scientific papers on prostate tumours – and five years ago had treatment for prostate cancer himself.
He says: "Like all men who have had surgery for prostate cancer, I took medication for erectile dysfunction afterwards. Although techniques have improved vastly over the years, surgery in that area will always damage the nerves responsible for erections.
"It takes time, but they do gradually recover. Taking Viagra, or a similar medication, helps that happen faster by stimulating blood supply to the area, which aids healing. That's why we suggest that men start taking it as soon as they stop feeling tender after surgery.
"I now tell patients that I recommend the same thing that I have done myself: a low dose of the drug every day with a larger dose as and when needed for activity.
"Five years on from my own operation, I only need to take the low daily dose and don't need the 'as and when' dose when my wife is feeling friendly – because my nerves have recovered sufficiently. Some men experience facial flushing and headaches, but I have found there are no real side effects. It's like popping an aspirin. You don't even realise you've taken it.
"Men are less embarrassed talking about the subject of erectile dysfunction now, especially if they're talking after prostate cancer. But of course there's a stigma. We don't want to admit vulnerability.
"I was actually involved in the early trails and launch of Viagra back in 1998, and there were scare stories at the time that it would bankrupt the NHS, turn people into sex fiends and trigger heart attacks.
"Of course, none of this came true, and we now know it's just a very safe, helpful drug that is very much part of 21st Century life."
NOT ALL WOMEN WANT THEIR MAN TO TAKE IT
Jo Coker is a psychosexual therapist and a spokeswoman for the College of Sexual Therapy and Relationships.
She says: "When Viagra launched, people thought it would be an instant solution to their flagging sex lives. But like all quick fixes and solutions in a pill, there are caveats.
"Let's not forget, there are two people in every couple and both need to be considered when drugs like this are being used. It can't be a decision that just one person takes.
"These days there is pressure on all of us to look great and have loads of sex – not everybody wants that, though. There are certainly some women who are relieved when the sexual side of a relationship slows down.
"Many hitting their 60s may be dealing with post-menopausal symptoms that mean sex is painful or their libido may have waned. The last thing they want is for the focus to be about sex.
"Time and again, I see couples who are on a different page sexually. But you need to look at why exactly you are thinking about taking a drug like Viagra. Is it because you want more sex or because you both want more sex?
"One thing that's certain is that taking a pill won't fix a broken relationship. Viagra is not going to change the fact your husband is bone-idle and lazy. Sex is just one part of the jigsaw."
IT WAS SUPPOSED TO BE A HEART PILL
Research scientist Dr Mike Wyllie was head of biology at the pharmaceutical company Pfizer during the development and launch of Viagra in the 1990s. He is still a consultant for the firm today.
He says: "Viagra – as it's used in its current form – was an accidental discovery. The drug was originally designed to help angina but an unexpected side effect was that the men trailing it were getting erections.
"As well as dilating their coronary blood vessels, as expected, the drug was acting on the blood vessels in the penis.
"And Viagra very nearly didn't happen at all. In fact it was very nearly shelved for being morally wrong: there were concerns over 'selling sex'. And very few people thought there was a market for it.
"Back in the early 1990s, erectile dysfunction, or impotence, as it was more commonly known, was not considered a problem. Or certainly not one that was ever talked about.
"And the vast majority of people thought it was 'all in the mind'. GPs said patients didn't mention it, but then they also never asked about it, not like today.
"Viagra's biggest achievement in the past 20 years has been to put the very embarrassing, but very widespread problem of erectile dysfunction on the map, and that can only be a good thing."
TODAY THERE ARE SO MANY OPTIONS FOR MEN
Lorraine Grover is a psychosexual nurse specialist based in London and Buckinghamshire and a trustee of the Sexual Advice Association.
She says: "One man in ten suffers from erectile dysfunction and many say Viagra has been fantastic for them. But this family of drugs, known as PDE5 inhibitors, which also include Cialis (tadalafil) and Levitra (vardenafil), only works for between about 70 per cent of men.
"Those who tend to get little or no effect after taking it often have nerve problems.
"Men with severe or uncontrolled diabetes can really struggle, as do those with low testosterone, particularly if they are on hormone treatment for prostate cancer.
"Then there are those who can't take it for health reasons.
"Many people also don't realise a man must be sexually aroused in order for Viagra to work. You don't suddenly spring into life an hour or two after taking it while reading the paper.
"But erectile dysfunction can wreck relationships and cause terrible depression, which is why it's so important for men know what other options are out there.
"Injections into the penis to relax the muscles, allowing blood to flow through before sex, are one option. This sounds like every man's worst nightmare but they really are effective. Vacuum pumps and constriction rings (to keep blood in the area) are another good alternative.
"For those who find pills work for them, but not at the right time, or who suffer from performance anxiety, one option is to take a small, 5mg daily dose of Cialis, available on private prescription. At full dose, the effects of Cialis last twice as long as Viagra.'
THE FUTURE: GELS, STEM CELLS AND SHOCKWAVES
David Ralph is a consultant urologist at University College Hospital London and in Harley Street. He is president of the Sexual Advice Association.
He says: "Viagra is the best option we have for erectile dysfunction. But there are promising treatments in the pipeline for men who can't take or tolerate the tablets, or get no effect from them. I have been involved in developing a new gel that provides an alternative, more localised treatment.
"The key ingredient is glyceryl trinitrate (GTN), a drug that has been used to treat angina sufferers in the past.
"It is applied to the penis and releases nitric oxide gas, which widens blood vessels and increases blood flow.
"Our research found that 44 per cent of men tested managed to achieve an erection after five minutes – 12 times faster than Viagra. It also avoided side effects such as headaches, and we hope it will be available to buy in the next couple of years.
"Another treatment growing in popularity is shockwave therapy.
"Each 15- to 20-minute outpatient session involves shockwaves applied to the penile shaft. This encourages new blood-vessel formation in the penis. Studies show it to be quite successful, and it can help people who don't respond to tablets.
"Patients have anywhere between four and 12 sessions. A course of treatment costs about £3,000 privately.
"From a research point of view, there is a lot going on with stem-cell therapy, where cells are injected into the genital area to encourage the growth of new blood vessels. This is still at a very experimental stage."