Viagra-fuelled over-45s having risky sex with people they meet through the internet are being blamed for a surge in sexually transmitted diseases among the middle-aged.

Married men in their fifties are the biggest risk-takers but women who are past worrying about pregnancy are also having unprotected sex without checking their partners' histories. When they catch something they are reluctant to confess to their spouses — or even to admit that anything is wrong — and so infections spread.

Researchers in the West Midlands report a doubling of infections among older people in the past decade and call for information campaigns that are not aimed simply at the young.

The team, led by Babatunde Olowukure, of the Health Protection Agency in Birmingham, counted the number of sexually transmitted infections (STIs) diagnosed in 19 clinics in the West Midlands between 1996 and 2003. The number rose steadily from 344 in 1996 to 780 in 2003, not including diagnoses of HIV/Aids. Genital warts was the most common diagnosis but the sharpest increases were seen for gonorrhea and syphilis.

In 1996 there were 23 cases of gonorrhea and none of syphilis among over-45s. By 2003 there were 152 cases of gonorrhea and 22 of syphilis. There were also 86 diagnoses of sex infections among over-60s in 2003.

The figures, published online in Sexually Transmitted Infections, are in line with national figures that show that between 2002 and 2006 cases of chlamydia among over-45s increased by more than 50 per cent and cases of syphilis by more than 100 per cent.

Sandra Gidley, the Liberal Democrat health spokeswoman, said of the national figures, published in April: “Most people think that STIs only affect young people but this is clearly not the case. The Government needs to ensure that information is available to all age groups, so that we can buck this worrying trend.”

The assumption has long been that the sexually active middle-aged are in low-risk monogamous relationships but one study has shown that 80 per cent of the over-50s are sexually active and 7 per cent engage in risky sex. The greatest risk-takers were married men aged between 50 and 60.

A recent survey by Saga found that one in ten sexually active over-50s did not use contraception and did not know a partner's sexual history.

The authors of the West Midlands survey say that older people are reluctant to admit that they have caught something and delay longer before visiting a doctor or clinic. “There is also growing evidence that the internet is being used to identify casual sexual partners by all age groups,” they say. “This is associated with increased high-risk behaviour.”

They say that the introduction of Viagra and other drugs for erectile dysfunction “has altered the quality of life and sexual experience of older individuals” and that the greater the age gap “the more likely it is that risky sexual behaviour will occur as condoms are less likely to be used”.

Julie Bentley, head of the Family Planning Agency, said: “We've noticed a rise in the numbers of over-45s phoning our helpline. Tragically, the sexual health of men and women of this age group is largely neglected.”

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