(CNN) The world's rivers are widely contaminated with antibiotics, according to a new global study, the first of its kind.

Researchers from the University of York in the UK analyzed samples from rivers in 72 countries and found that antibiotics were present in 65% of them.

Dangerous levels of contamination were most frequently found in Asia and Africa, the team said, with sites in Bangladesh, Kenya, Ghana, Pakistan and Nigeria exceeding safe levels by the greatest degree.

The worst case was found at a site in Bangladesh, where concentrations of the drug Metronidazole -- which is used to treat bacterial infections, including skin and mouth infections -- exceeded safe levels by up to 300 times.

Safe levels can range from 20,000 to 32,000 nanograms per liter (ng/l), depending on the antibiotic, according to new guidelines established by the AMR Industry Alliance, a coalition of biotech, diagnostics and pharmaceutical companies set up to provide sustainable solutions to curb antimicrobial resistance.