An estimate published online Wednesday by The Lancet says that some 16 million people around the world inject illegal drugs, and of them nearly one in five may have the AIDS virus.
The global tally of intravenous drug users (IDUs) is put at 15.9 million, around three million of whom could have the human immunodeficiency virus (HIV), it says.
China has largest number of IDUs, with a mid-range estimate of 2.35 million people. The HIV infection rate among them is calculated at 12.3 percent.
The United States has the second highest total, with around 1.85 million IDUs and an estimated infection rate among them of between 15.6 percent.
The report also warned of high HIV numbers among IDUs in Ukraine and Russia, which could be 42 percent and 37 percent respectively.
The assessment is led by Bradley Mathers of the National Drug and Alcohol Research Centre at the University of New South Wales in Sydney, Australia.
It is based on official national figures and estimates published in peer-reviewed journals.
The review covers 148 countries, but admits that many blanks remain where the data are sketchy or absent and the range estimates are broad.
"Areas of particular concern are countries in Southeast Asia, Eastern Europe and Latin America, where the prevalence of HIV among some sub-populations of people who inject drugs has been reported to be over 40 percent," the paper says.
Injecting drug use is one of the major drivers for the global AIDS pandemic.
HIV is spread by infected IDUs who share syringes or turn to prostitution, which thus helps the virus to enter the main population.
AIDS campaigners say the problem has to be tackled by a panoply of methods, including programmes to exchange used needles for sterile ones and the use of methadone, an opiate substitute, to wean IDUs off heroin.