They can substantially reduce the risk of heart attacks and strokes, may stave off dementia, and could even boost your sex life.

With such a myriad of benefits, it's no surprise that cholesterol-lowering statins have been hailed as wonder drugs.

Indeed, the health watchdog NICE (National Institute for Health and Care Excellence) recently issued draft guidance recommending that the drugs are offered to "many more people" than the seven million Britons who currently take them (statins are the most widely-prescribed drug in the UK), pointing out that "the effectiveness of these medicines is now well proven and their cost has fallen". Final guidelines are due to be published next month.

In addition, the Medicines and Healthcare products Regulatory Agency (MHRA) has just confirmed that people should carry on taking prescribed statins because "their benefits continue to outweigh the risks of any side effects".

However, it's rare to hear of any drug that has no critics, and statins are no exception.

While statements published in the British Medical Journal (BMJ) last autumn suggesting 'adverse events' from taking statins occur in 18-20% of patients have now been withdrawn, the drugs still have their detractors.

One such critic is GP Dr Malcolm Kendrick, author of The Great Cholesterol Con (John Blake Publishing, 2007), who insists that adverse events caused by statins, including muscle pain, irritability, abdominal pain and skin problems, mean that while the drugs "are moderately good at reducing the likelihood of dying from heart disease, in anybody else, the downsides outweigh any benefits".

So what's the evidence for and against these pills?

They've certainly been found to cut the risk of heart attack and stroke hugely - the world's largest study of cholesterol-lowering drugs at Oxford University concluded that taking a statin a day reduces such risk by a third.

This is linked to the way statins inhibit an enzyme involved in the synthesis of cholesterol in the liver, thus reducing the chance of artery blockages and associated cardiovascular problems.

But while statins are prescribed for this purpose, studies have suggested that they also have many other benefits.

In April, Taiwanese researchers found that patients prescribed statins were 22% less likely to suffer dementia than those who never took them.

It's also claimed that the drugs could ease some MS symptoms, and protect diabetics against cardiovascular disease - although some studies have suggested an association between statins and a slightly increased risk of developing diabetes.

They may even improve your sex life - earlier this year, US researchers found that statins can reduce the problems of erectile dysfunction.

Professor Peter Sever, from Imperial College London's International Centre for Circulatory Health, has been involved in several studies on the effects of statins, and says: "I think statins are probably one of the safest groups of drugs we've ever been fortunate enough to prescribe to a huge worldwide population.

"Stroke and heart disease are the world's most common cause of death and disability, and here we have a drug which has been shown in probably millions of patients to be largely safe and extremely effective.

"It's affordable, and cost effectiveness analyses have shown that it's one of the best and effective interventions that we have in modern medicine."

Sever, a professor of clinical pharmacology and therapeutics, led the Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT), in which 5,000 patients took statins and 5,000 took a placebo.

The statins group were found to have a third reduced risk of heart attack and a quarter reduced risk of stroke.

"But you could not tell from the side effect profile who was on statins and who was on placebo," says Sever. "That's been a consistent finding in all the statin trials.

"The incidence of myalgia [muscle pain] is almost exactly the same on the placebo as it is on the active drug."

He suggests it may be that people taking a tablet that they think is a statin complain of myalgia, "but when you look at the people who complain, they're older people who get muscle aches, or people who exercise and get more muscle aches", he notes.

"They're just as likely to develop them if they're on a placebo, as they are if they're on a statin, and that's the only way you can explain it."

He believes that stories in the media have created a "scare factor" about statins, pointing out: "People believe they're going to get side effects because they've read all about them in the newspapers.

"You've only got to be on a pill for a few weeks, go for a walk and get a muscle ache, and you attribute it to the drug when that's not necessarily what it was due to."

Stressing that people should continue to take their statins as prescribed, the MHRA says evidence has shown that the drugs can save lives, but like all medicines, they can cause side effects in some people.

Most are mild, muscle-related problems, and Dr June Raine, the MHRA's director of vigilance and risk management of medicines, says: "Large clinical trials have shown that statins can save lives by reducing the risk of heart attacks, strokes and the need for heart surgery.

"The MHRA continually reviews the information on the safety of statins. We will update the prescribing advice for statins in light of any substantial new evidence of safety concerns."

She says if patients have any worries about their medication, they should speak to a doctor, and report any perceived side effects through the MHRA's Yellow Card Scheme (www.yellowcard.mhra.gov.uk).

Dr Malcolm Kendrick says the most common adverse effects from statins are muscle and joint pains, weakness and difficulty standing.

The GP says there may also be irritability and anger, and cognitive effects which make concentrating and remembering difficult, possibly even mimicking the onset of dementia. Upset stomachs and abdominal pain can also occur, he says, as can skin problems, such as eczema, dermatitis and rashes.

"There's a really wide spectrum of problems," Kendrick adds, "and I tend to tell patients that statins add 15 years to your life - they don't make you live 15 years longer, but they make you feel 15 years older."

He insists that adverse effects from statins are "extremely common" and claims that pharmaceutical companies don't release data on the topic, while clinical studies are carried out on "artificial groups of people" who don't reflect the type of patient who'd take statins in real life.

"If you've already had a heart attack or a stroke, or are at very high risk of them, then I think it's worth taking a statin," he says. "But be careful, and look for the adverse effects."

If people on statins do begin experiencing typical side effects, he believes it's "quite likely that it's the statin that's causing it".

"And then you have to decide whether you continue taking it or not - are the benefits great enough to outweigh the reduction in your quality of life?" Kendrick continues.

"Anecdotally, and in my own experience of dealing with people, I've seen an awful lot of people with statin-related adverse effects, some of which have been very serious, so I know the rates must be higher than are reported in the clinical trials."