The number of teenage boys who have visited prostitutes is on the rise, a trend that has researchers worried as some do not use condoms consistently, may catch sexually transmitted diseases and go on to infect their girlfriends.

In the first-ever study of Singapore youth who have had sex with prostitutes, it was found that about two in five of the teens surveyed had paid for sex. The interviews were conducted with some 300 heterosexual boys aged between 16 and 19 who went to a government specialist clinic that treats sexually transmitted infections (STIs), between 2009 and 2014.

In earlier cohorts of boys who went to the Department of Sexually Transmitted Infections Control (DSC) Clinic between 2006 and 2009 and were interviewed, only 15 per cent said they had intercourse with prostitutes.

Associate Professor Wong Mee Lian - working with Adjunct Professor Roy Chan and Dr Martin Chio at the clinic - invited new teenage patients at the clinic each year since 2006 to be part of their research.

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The doctors wanted to understand their background and behaviour which could put them at risk of contracting an STI, and find ways to reduce infection rates among the young.

Prof Wong, who is with the Saw Swee Hock School of Public Health at the National University of Singapore, said the jump in the proportion of teens who visited prostitutes is significant.

While the findings cannot be generalised to represent the population of teens who are sexually active, she said they do suggest that it has become more common recently for teenage boys to pay for sex.

This is a public health concern as sex workers are a major source of infection - and 30 per cent of the boys she polled between 2009 and 2014 who visited prostitutes said they did not use a condom each time they had paid sex.

The worry is that they could infect their girlfriends or others they sleep with, she said, as they had a median of 4.5 sexual partners, including their girlfriends, by the time they were polled. "Many of those infected with an STI show no symptoms and they could have sex with others without knowing they are infected," she said.

Her study was published in the science and medical journal PLOS One in January. PhD student Junice Ng was the co-author.

Another worrying finding is that of those who had paid for sex, their median age was only 16 and 38 per cent had their first sexual encounter with a sex worker.

Doctors and social workers say two main factors account for why more teens visit prostitutes. The first is the easy access to pornography online. There has also been a proliferation of vice websites advertising sexual services here.

Dr Lin Kai Wei of Nuffield Medical Siglap clinic said his young patients go online to seek sexual services as they find it less daunting than going to the red light districts where others can see what they are up to. "There are all these websites where there are pictures of the girls and their vital statistics. So the boys are tempted," he said.

Singapore Children's Society chief executive Alfred Tan, a father of two, including a 21-year-old son, said the study's findings are worrying, especially as youth are more exposed now to all sorts of vice online.

"We have to start talking about sexuality issues with our children, to let them know what's right and what's wrong, when they are as young as possible," he said.