A saviour a killer ,an economic liberator. The Pills is 40.

The Pill-a device that some think is the single most defining invention of the 20th century and others see as nothing more than a four-letter word. No other drug in the history of medicine has been so examined, caused more enthusiasm, fright and fury. The name itself seems loaded with significance. Singular, mysteriously capitalised: of all the tablets and potions we take, this is the Pill.

Of the 11 million women of reproductive age in the UK, an estimated three million are on the Pill. To users of the drug, the little rattling packet is as familiar as their toothbrush. Familiar as the Pill is, an uneasiness with it remains, a lurking fear that it must carry with it some dangerous catch. But the Pill a woman takes today is not the same as the one her mother took. It is far safer. It also has the power to protect against certain deadly diseases. In ways that the drug\'s inventors could not have anticipated, the Pill has benefits that go far beyond simple birth control. Put simply, it can save your life.

And the reason it can do this is perhaps one of the greatest medical ironies of the 20th century.

From the start, the very idea of the Pill was troublesome. To give a drug every single day for years to a large, perfectly healthy population of young women seemed to go against most people\'s notions of how-and why-drugs should be used. The majority of women do not take the Pill to ward off life-threatening disorders, or to alleviate pain, or to control unpleasant symptoms caused by unavoidable disease. Most women take the Pill because they wish to have sex. Sex without worry, without the fuss and uncertainty of condoms and diaphragms.

The concept of sex without worry took a serious blow with the arrival of Aids, but women in monogamous relationships still take the Pill so that they can enjoy a sexual freedom unknown to previous generations. It\'s hardly surprising that the drug has always inspired a mixture of love and loathing. Carl Djerassi, the chemist who first synthesis-ed a steroid oral contraceptive in 1951 and who is sometimes described as "the father" of the Pill, also links our ambivalence towards the drug to the era in which it was first introduced to society:" Till the late 60s, the three most important social movements were the women\'s movement, the rise of environmental protectionism and consumer advocacy. All were immensely beneficial, but all were, to some extent, suspicious of technology, almost anti-science."

But although the Pill became one of the symbols of the 60s, its creation was influenced far more by the cultural concerns of the previous decade. When it was invented, the Pill was designed to be taken in a cycle consisting of three pill-taking weeks, followed by seven "free" days to allow a woman an episode of bleeding that corresponded to normal menstruation. This is still the method used today. But few Pill-users know that there is no real medical justification for taking it this way.

In theory, the drug might have been designed to free women from the obligation of having a period every single month. The decision to create a method of birth control that mimicked women\'s normal menstrual cycles was based on a patronising assumption-that women wanted to continue having periods every single month. That it was comforting to them. That there was, in short, a need to bleed.

The Pill was developed by a team of scientists, of whom the leading figures were Carl Djerassi, biologist Gregory Pincus, who demonstrated the drug\'s efficacy on animals, and Harvard-based gynaecologist John Rock, who conducted the clinical trials on humans. The journalist Malcolm Gladwell, writing in the New Yorker, suggests that the form the Pill took, right from the start, was influenced, to a large degree, by Rock\'s belief that the drug was a "natural" form of birth control. The Pill contains synthetic versions of two naturally occurring hormones, progesterone and oestrogen. In normal circumstances, the levels of these hormones fluctuate throughout a woman\'s menstrual cycle, resulting in the release of an egg from the ovary and the growth of the uterine lining in preparation for the fertilised egg. On the Pill, however, the hormones are given in a steady dose, preventing ovulation from occurring.

To Rock . What was natural about the Pill was that it did not introduce anything into a woman\'s body that didn\'t already exist there, albeit in a more intermittent fashion. The great irony is that, in the years since the Pill was invented, what used to seem normal and "natural" is revealing itself to be just the opposite. John Rock thought he was maintaining a status quo by allowing women their monthly period. What he did not know was that later research would fundamentally challenge this status quo. For while it is true that modern women are used to menstruating once a month, the same cannot be said of their ancestors. It now appears that far from being natural, constant menstruation is an abnormality in evolutionary terms.

During the 80s, scientist Beverly Strassmann studied the reproductive life of women in the Dogon, an African tribe whose lifestyle had remained largely unchanged since antiquity. She wanted to form a picture of what women\'s biological patterns might have been like before the advent of modernity. What she discovered was that Dogon women spent most of their fertile years either pregnant or breastfeeding, both of which suppressed their ovulation for long periods of time. As a result, they only experienced, on average, 100 menstrual periods before they reached menopause. By contrast, the average contraceptive women in the West has between 300 and 400 periods.

The division and reproduction of cells necessitated by ovulation and the growth of a woman\'s uterine lining increases the risk of ovarian and womb cancers. By lessening the frequency of that cell activity, the Pill increases a woman\'s chances of avoiding these diseases. The advantages are significant. A woman who takes the Pill for 10 years reduces her risk of getting cancer of the womb by about 60 percent, and her risk of ovarian cancer by around 70 percent. These benefits of the Pill have been known about for decades, but they remain among the most unsung of the drug\'s side-effects.

Meanwhile, health scares surrounding the Pill continue to make headlines. When the drug first came out, concerns over its safety proved justified. Serious and even fatal complications occurred among users, prompting more research. Since then, hormone doses in the Pill have been drastically reduced. Today, there is only one hundredth the amount of oestrogen that was in the original. Other modifications have added to its safety. During the 80s, a triphasic version of the drug was introduced, providing three different levels of hormone to be taken over the course of a month and corresponding more closely to a woman\'s natural cycle. Despite the safety of the new Pill, however, fears persist.

In the mid-90s a link was suggested between the third-generation Pill and deep-vein thrombosis. Much was made of the fact that women taking this version had double the chances of death over users of the second-generation Pill.

"Anybody who drives on the roads for a single hour per year increases their risk of death to one in a million," claims John Guillebaud, professor of family planning and reproductive health at University College, London, and author of The Pill (OUP, fifth ed 1997). "The difference in risk between the two brands of Pill is the same as going from two hours of driving per year to four."

Recently, a suggested link between the Pill and breast cancer has been causing debate, and here the risks appear less clear-cut. At the moment, there\'s no hard evidence that the drug definitely increases a woman\'s chances of developing the disease.

Why, after four decades of its use, is the Pill still demonised? Why do the fears persist out of all proportion to actual risk? "When I\'m counselling women about whether they should take the Pill," says Sheila Hancock, a nurse with Britain\'s Marie Stopes clinics who has been working in family planning since 1977, "my goal is to give them all the information to date and let them decide. But most of the women I see have far more knowledge about the negative aspects of the drug. Very few know about the cancer benefits. I do wonder sometimes about why we go on having these terrible scares. Perhaps society as a whole is saying, \'Oh no, the Pill, that\'s naughty...\'"

"It\'s to do with sex," agrees Guillebaud. "People still have hang-ups. They can\'t believe nature won\'t bite back in some way."