Fake and poor quality anti-malarial drugs are threatening efforts to control the disease in Africa and could put millions of lives at risk, scientists say.
The counterfeit medicines could harm patients and promote drug resistance among malaria parasites, warns the study, funded by the Wellcome Trust.
Malaria is believed to kill about 800,000 people a year.
Some of the fake tablets are said to have originated in China.
The researchers, from the Wellcome Trust-Mahosot Hospital-Oxford University Tropical Medicine Research Collaboration, published their work in the Malaria Journal.
They examined fake and substandard anti-malarial drugs that were found on sale in 11 African countries between 2002 and 2010.
They discovered that some counterfeits contained a mixture of the wrong pharmaceutical ingredients which would initially alleviate the symptoms of malaria but would not cure it.
Some of the ingredients in the tablets could cause potentially serious side effects, the study found, especially if they were mixed with other drugs a patient might be taking, like anti-retrovirals to treat HIV.
The malaria parasite can, after a period of time, develop resistance to the drugs being used to treat it.
This has happened in the past with medicines such as chloroquine and mefloquine.
The researchers warn that the fake drugs could lead to the same effect on artemisinin, one of the most effective drugs now being used to treat malaria.
They say small quantities of artemisinin derivatives are being put in some of the counterfeit products to ensure that they pass authenticity tests.
However, at the level it is present, these drugs are unlikely to rid the body of malaria parasites, but could enable them to build up resistance to artemisinin, the study warns.
The Asian origin of the fake drugs was identified using traces of pollen found in some of the tablets.
The lead researcher on the study, Dr Paul Newton, called for urgent measures from African governments to tackle counterfeit anti-malarials.
"Failure to take action will put at risk the lives of millions of people, particularly children and pregnant women," he said.
"The enormous investment in the development, evaluation and deployment of anti-malarials is wasted if the medicines that patients actually take are, due to criminality or carelessness, of poor quality and do not cure."