Valentine's Day on February 14 is the usual time of year for Thai authorities to warn against premarital sex, but this time the emphasis is on protection rather than prohibition.
Most Thais take a conservative view of sex, despite a massive commercial industry centred on the famed red light areas of places such as Bangkok, the capital, and the beach resort of Pattaya.
But rapid urbanisation is leading more young people to have their first sexual experiences earlier, health officials say.
"Teenagers, especially, do not have to be embarrassed about buying condoms," said Thongchai Lertwilairattanapong of the Ministry of Public Health.
"Society also has to accept that teenage girls buy condoms, which is better than more teenage girls getting pregnant."
Thailand has one of the highest rates of teenage pregnancy in Southeast Asia, the Bureau of Reproductive Health says, with the number of teenage mothers second only to neighbouring Laos.
"We have to persuade Thais to accept condoms as a hygiene item in everyday life to protect against pregnancy and AIDS," said Panumard Yarnwaidsakul, deputy director-general of the Department of Disease Control.
The campaign launch coincided with Thai lawmakers' approval of a measure aimed at preventing teenage pregnancy.
After sub-Saharan Africa, Asia-Pacific is the region with the largest number of people living with HIV, and Thailand accounts for about 9 percent of that number, says HIV response outfit AVERT.
About 450,000 people live with HIV among a population of 67 million, according to health ministry estimates from 2013, the latest available data.
As many as 90 percent of the 7,800 new infections reported that year were contracted by those who had sex without protection.
Health authorities distribute about 40 million free condoms every year.
A free condom campaign in the early 1990s helped to rapidly reduce new cases of HIV infection which had reached about one million by 1993, according to the World Health Organisation.
The new campaign, set to run until 2019, is not the first time Thailand has tried to woo customers too shy to buy condoms in the shops. In 2007, Kasikorn Bank organised a distribution campaign, "Condoms for Confidence", to alert people to infection risks.
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