Substandard tuberculosis drugs sold by pharmacies in poor countries are a growing public health threat, a new study reports.

At pharmacies in 17 countries, the authors bought 713 samples of two TB drugs, the antibiotics rifampin and isoniazid. Nine percent had no active ingredient or worse, too little: An inadequate dose encourages the growth of drug-resistant TB strains while not curing the patient.

The study, published online by PLoS Medicine, shows the failures in what should be a nearly perfect system for basic TB control, said Dr. Lucica Ditiu, executive secretary of the Stop TB Partnership.