Many pills released the active ingredient too slowly. Others had the wrong active ingredient. One batch had no active ingredient at all, researchers said.
To detect the fake drugs, researchers at the Georgia Institute of Technology developed a sophisticated approach using mass spectrometry to quickly assess suspected counterfeit drugs and then characterise their chemical composition.
The study's results highlight a growing concern for women's health in developing nations.
"A woman who does not want to get pregnant and takes these emergency contraceptives will get pregnant," said Facundo M Fernandez, a professor in the School of Chemistry and Biochemistry, whose lab investigated the contraceptives.
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Recent research has found that falsified drugs are a major problem in developing countries, researchers said.
Falsified emergency contraceptives have been reported in Nigeria, Ghana, Kenya, Angola, South America and even the US. Fake drug manufacturers will copy everything from the pill to the package.
Just as concerning as counterfeit medications are other poor quality medications, such as degraded or substandard drugs.
Degraded drugs were once good quality, but lost their efficacy over time, for example after prolonged exposure to the Sun in an open air market, researchers said.
Falsified drugs are the most worrisome, because they may not contain the expected active ingredient, or they may contain the wrong ingredients, including toxic compounds.
In the survey of emergency contraceptives from Peru, the researchers found that seven of the 25 batches analysed had inadequate release of the active ingredient (levonorgestrel). One batch had no detectable level of the active ingredient.
"We detected that the active ingredient was not there in one batch, instead those samples had a drug called sulfamethoxazole," Fernandez said.
The study sponsored by the ACT Consortium through a grant from the Bill and Melinda Gates Foundation was published in the journal PLOS ONE.