A survey produced such dramatic results that lead researcher Dr. Irwin Goldstein advised any woman on the Pill who has sexual problems to stop taking it and try another method of birth control.

"There is a possibility it is imprinting a woman for the rest of her life," he said.

The Pill was launched in Britain at the beginning of the Swinging Sixties with the promise of freeing women from the fear of unwanted pregnancy.

Despite a number of health scares, three million still take it.

But now it appears they could pay a worrying price. Synthetic hormones in the Pill which stop ovulation also appear dramatically to lower levels of the hormone testosterone, which is vital to both male and female libido.

'When Pills Are Handed Out Like Candy'

When women on the Pill were tested, levels of a chemical which wipes out testosterone were found to be seven times higher than in those who have never taken it.

Most worryingly, even those who were not on the Pill but had taken it in the past had levels up to four times higher than those who had never used it.

Past studies had suggested that taking the Pill can dampen a woman's sexual desire but if she comes off it her libido returns within four weeks.

Dr. Goldstein, former director of the Institute for Sexual Medicine at Boston University, Massachusetts, said that while his research seemed to suggest the effects could be permanent, more investigations were needed to establish this for certain.

His colleague Dr. Claudia Panzer said: "When Pills are handed out like candy I do not think doctors always tell women about potential side effects. I don't think women realize the Pill can do bad things to their sex life. Not everybody will be affected, but this is out there and it is real. Women should know it can have effects on their sex lives and they should be told the pros and the cons."

Many Factors Affect Desire

The research, revealed in New Scientist magazine, took 125 women in their mid-30s who were attending a clinic for sexual dysfunction.

Sixty-two were taking the Pill, 40 had taken it in the past but were no longer doing so, and 23 had never taken it.

All those who were on the Pill or had taken it in the past had been on it for at least six months.

The study team, who tested the women every three months for a year, measured levels of a chemical called Sex Hormone Binding Globulin (SHBG) which takes testosterone out of circulation.

They found that levels of SHBG were seven times higher in Pill users than in those who had never taken the oral contraceptive.

Among those who had taken it in the past but stopped, levels were still three to four times higher. In previous research, Dr. Goldstein found the Pill can leave a woman with only a third of the testosterone levels she needs to enjoy sex.

However, he also pointed out that there are many factors which affect desire, and if a woman trusts her method of contraception, that could be a bigger factor in her wanting sex than elevated levels of SHBG.

Some Brands Actually Boost Testosterone Levels

A spokesman for the fpa, formerly the Family Planning Association, said the researchers had not provided enough details about other factors which might have affected the women's libido.

She said not all brands of Pill lower testosterone and some actually boost levels, which is why certain women can suffer side-effects such as acne.

"Studies do show that for some women, using the contraceptive pill or other hormonal methods may affect mood change and interest in sex," she said. "In most cases, where other factors are not considered to be the problem, changing pills or using another effective non-hormonal method is advised.

"The issues around sexual interest and non-interest are highly complex. Use of hormonal methods generally are on the increase which suggests that loss of libido is not an issue for most women."