Africa, India and other developing countries are awash in fake or sub-standard drugs for tuberculosis, fuelling the rise of treatment-resistant strains of TB, according to a survey published on Tuesday.
Investigators in the United States asked local people in 19 cities in 17 countries to purchase isoniazid and rifampicin, the frontline antibiotics for TB, from a private-sector pharmacy.
The samples were then examined by chromatography, a technique that detects chemical signature, for their active ingredient.
They were also tested for disintegration, to see if they properly broke up in water at body temperature within 30 minutes.
Out of 713 samples, 9.1 percent failed these basic quality control tests, according to the probe, published in the International Journal of Tuberculosis and Lung Disease
Around half of the failed samples had zero active ingredients, "making them likely to contribute to drug resistance," it said.
Resistance to TB drugs develops when treatment fails to kill the bacteria that causes it - either because the patient fails to follow their prescribed dosages or, as in this case, the drug doesn't work.
It can also be contracted through rare forms of the disease that are directly transmissible from person to person.
Dud drugs were manufactured by legitimate companies and criminal fraudsters, said the report.
The pharmacies where the drugs were purchased were in Luanda, Angola; Sao Paulo, Brazil; Beijing, China; Lubumbashi, Democratic Republic of Congo; Cairo, Egypt; Addis Ababa, Ethiopia; Accra, Ghana; Chennai, Delhi and Kolkata, India; Nairobi, Kenya; Lagos, Nigeria; Moscow, Russia; Kigali, Rwanda; Dar-es-Salaam, Tanzania; Bangkok, Thailand; Istanbul, Turkey; Kampala, Uganda; and Lusaka, Zambia.
The failure rate was 16.6 percent in Africa, 10.1 percent in India and 3.9 percent in Brazil, China, Thailand, Turkey and Russia.
Nearly nine million people around the world have TB, including more than 400,000 with a multidrug-resistant form of the disease, according to estimates for 2011 compiled by the World Health Organisation (WHO).
TB is one of the world's deadliest diseases. It is spread from person to person through the air and usually affects the lungs, but it can also affect other parts of the body such as the brain and kidneys.