The nation's 16 and 17-year-olds are having less sex but using condoms more, says the Thai Health Promotion Foundation.

The findings come from surveys of second-year vocational and Mathayom 5 (grade 11) students between 2015 and 2017, said Chartwut Wangwol, director of the foundation's health risk control office.

Second-year vocational students and Mathayom 5 students of both sexes took the survey. In 2017, on average 42% of male and female vocational students had sex, down from nearly 50% in both sexes the year before. In 2017, 25% of Mathayom 5 male students and 17% of female students had sex. The numbers were almost the same in the year before. However, more students are using condoms.

In 2017, 69% of male vocational students used condoms when they had sex for the first time, compared with 74% of female students who asked their partners to use condoms. In the same year, 75% of male students in Mathayom 5 used condoms during their first sexual encounter while 77% of female students asked their partners to use condoms. The foundation did not give comparable figures for the previous year of 2016 but confirmed there was an increase in the use of condoms.

Mr Chartwut said the condoms were not being used consistently by students. The reason given was that students stopped using condoms as they developed trust after having gone "steady" with their partners for a while. This contributed to a proliferation of sexually transmitted diseases (STDs) such as HIV/Aids, syphilis and gonorrhea, he said.