The 30-year-old businessman was on a working trip in Bangkok. He visited a male prostitute on his last night in the city. Against his better judgment, they had unprotected sex

But the next morning, he was seized by panic with the sudden realisation he might have exposed himself to HIV - the retrovirus that can lead to Aids

Upon landing in Singapore that afternoon, he headed straight for the Department of Sexually Transmitted Infections Control

Not only did he get an Aids test, but he also asked to be prescribed post-exposure prophylaxis (PEP)

He had read that the cocktail of antiretroviral drugs with Zidovudine works like a 'morning after' HIV prevention course and may keep the virus at bay

He breathed a sigh of relief when another HIV test three months later pronounced him clean

Studies have shown that PEP is not 100 per cent foolproof against HIV. Its efficacy is around 81 per cent

cocktail is best taken as early as possible - not beyond the window period of 72 hours after exposure