Intravenous drug use is helping fuel the explosion of HIV and Aids in South-East Asia, according to a report for the World Bank.
Cheap heroin supplies, poverty and a lack of effective law enforcement are responsible for the trend, officials said.
The Thai authorities have already significantly reduced the number of people infected through using prostitutes, but the report warned that little is being done to halt the spread of the disease among drug users.
The report estimated that a quarter of all new HIV cases in Thailand were drug users passing the infection onto their partners by having unprotected sex.
In Bangkok, HIV infection rates rose from 2% of intravenous drug users 10 years ago to 40% today.
The report wants the Thai Government to do more to control drug use, by helping addicts deal with their addiction and discouraging sharing needles.
In neighbouring countries such as China, Laos and Malaysia, it is illegal to wean addicts off hard drugs using substances such as methadone.
This, the study said, could be a significant factor in the promoting the further spread of AIDS.
But it is not a lost cause. Chris Beyrer, director of the US-based
John Hopkins University AIDS Training Programme who contributed to the
study said: HIV prevalence among drug users is not a hopeless
problem.
Strong government action has proven successful in Sydney, Glasgow
and other cities,
he said.