The countries' proximity to Afghanistan -- the world's top producer of heroin and opium -- is considered the major factor behind the mounting problem.
Other factors, such as poverty and high unemployment, are also fueling the drug trade and fostering drug abuse in the region.
As a result, addiction is growing especially rapidly among the region's youth. Reports show the age at which people start taking drugs is dropping. The number of women addicts is also growing.
Murtazokul Khidirov is the director of RAN, an NGO providing counseling and clean syringes to drug addicts in Dushanbe, the capital of Tajikistan.
"In Dushanbe, about 30 percent of the drug users are women. [Some of] the women engage in prostitution to get money [for drugs]," Khidirov said.
A 2002 report provides the UN's latest estimates on the number of drug users in the region. According to the report -- based on an assessment launched in 2000 -- the number of drug addicts in Kazakhstan, a country of some 15 million, is estimated to be as high as 186,000.
Neighboring Kyrgyzstan is estimated to have as many as 100,000 drug users out of 5 million people. The number of drug users in Tajikistan, a country of 6 million, is believed to be as high as 55,000. In Uzbekistan, the most populous Central Asia country with 25 million people, the number was estimated at between 65,000 and 91,000.
No figures were available on Turkmenistan. The International Narcotics Control Board (INCB) recently criticized Turkmenistan for lack of cooperation with the international community in its fight against illicit drugs. But the UN says that Turkmenistan has begun to cooperate in a number of regional programs.
James Callahan, the regional director of the UN Office on Drugs and Crime (UNODC), told RFE/RL that the drug problem has worsened in the region, particularly over the past two years: "We haven't had any assessment since 2000, and we rely entirely on government information in the meantime. Government information from the governments in the regions that report [on drug use] indicates that they feel the abuse situation has more or less stabilized over the last few years. I think that the feeling, though, is that, anecdotally, the situation is continuing to deteriorate or get worse again because of the increasing traffic from Afghanistan."
Cultivation of opium -- the raw material for heroin -- was banned in Afghanistan under the five-year rule of the Taliban militia. But since the fall of the regime following the U.S.-led invasion in 2001, heroin production has once again rocketed in Afghanistan.
Heroin now constitutes about 70 percent of the drugs seized in Central Asia. Its relatively cheap price -- a dose costs no more than $1 to $2 -- also means it has become the drug of choice among addicts.
An additional problem is that heroin addicts in Central Asia use the drug intravenously -- something that has contributed to one of the world's fastest-growing infection rates of HIV, the virus that causes AIDS.
Murtazokul Khidirov of Tajikistan's RAN drug organization said a lack of social and economic opportunities is drawing Central Asian youths into the drug culture.
"In Tajikistan, as you know, there was a civil war, and during this war all the economic structures were destroyed. On the other hand, there are a lot of narcotics being trafficked into Tajikistan from Afghanistan. And apart from that, the social life has been destroyed so it can't be better than this," Khidirov said.
David Lewis, the Central Asia project director for the International Crisis Group, said mounting despair is prevalent among the region's young people: "A lot of young people are looking for ways of escape from what is a very difficult reality: high levels of unemployment, [few economic] opportunities, difficulties in getting a good education. And of course, the drugs are relatively cheap in Central Asia."
In a report in 2003, the International Crisis Group said "drugs have become part of the lifestyle of Central Asian youth, not only because they carry a commanding image of power and of being cool, but also because they mean money."
Some programs have been launched to combat drug abuse and treat addicts. But experts say most of the efforts have been focused on penalizing drug users rather than treating them for their addiction.
Kyrgyzstan in 2000 became the first country in the region to introduce the use of methadone as a drug substitute for recovering addicts. On a local level, some NGOs are also involved in the fight against drug use.
But Lewis said these are only small steps and that there is a pressing need for more government commitment to drug-treatment programs.
"In some countries there have been attempts to work with drug users, to provide advice," Lewis said. "In some cases there have been drug replacement programs such as the use of methadone, there have been attempts to provide needle-exchange programs to prevent the spread of HIV. But these are rather small steps, and some of the governments in the region have not perhaps taken the issue seriously enough."
Lewis added that in some cases, drug addicts are reluctant to use even those services that are available because they fear being detained by police or being publicly identified as a drug user.
The international rights watchdog Human Rights Watch in 2003 criticized Kazakh authorities for targeting and harassing drug users.
A 27-year-old drug addict in the city of Almaty said he is trying to give up his habit: "I want to get married and be a parent, like everybody else. Life is hard. But if a drug addict doesn't long for life, then the end will be either jail or death. I don't want to die for nothing. And one more thing: Of all the drug addicts, only 10 percent are able to give it up. The rest are not able to do so. Those 10 percent are real fighters, sticking to life, really intending to survive. As you see, I am alive now, I am doing my best to become one of those 10 percent."
(RFE/RL's Kazakh Service contributed to this report. The second part of this series looks at the social and economic impact of the drug trade.)