Researchers have warned that substandard medications are available in countries across the world.
The medication is given after a heart attack
Heart experts looked at samples of a clot-busting drug sold in countries outside the European Union.
Testing found only three out of 16 samples tested met minimum EU requirements.
Writing in the European Heart Journal, they warned more global awareness of the problem of substandard drugs was needed.
The UK's Medicines and Healthcare Products Regulatory Authority (MHRA) said that a strict system of checks ensured that drugs marketed to the NHS met certain standards.
She said counterfeit drugs did occasionally come to light, but they tended to be of more saleable products such as Viagra.
The warning over the availability of substandard drugs around the world came following tests carried out by German researchers.
A team from ZLB Behring GmbH in Marburg tested 16 versions of streptokinase - a clot-busting drug used in the emergency treatment of heart attacks to see if they contained enough streptokinase to be effective.
If it is administered within an hour of a heart attack, an effective dose of streptokinase can cut the risk of dying by over a quarter.
The drugs had been on the market in several non-EU European countries, Brazil, India, Jordan, China and Pakistan.
They found only three, including one made by ZLB Behring GmbH matched European requirements for the quality and safety of medicines.
The activity of the remainder varied from 86% of the claim on the label right down to just a fifth. The European regulator, the European Pharmacopaeia requires activity of between 90 to 111%.
The analysis also found differences in composition and purity.
Dr Peter Hermentin, who carried out the tests, said: "The discrepancy between claimed and actual activity of many streptokinase preparations could cause life-threatening situations in severely ill patients."
Writing in the European Heart Journal, Dr Felicita Andreotti of the Institute of Cardiology at the Catholic University Medical School in Rome, Italy, said: "It's essential that we acknowledge the problem of counterfeit and substandard drugs and recognise it without embarrassment.
"It is a serious problem, particularly in the case of life-saving medications used to treat common diseases.
"Regardless of whereabouts, it would be a pity if emergency treatment were useless. Getting this right is a global responsibility."
She added: "The World Health Organization estimates that up to a tenth of the world's drug trade and a quarter in developing countries consists of fakes."
A spokeswoman for the MHRA said: "We have seen problems with counterfeit drugs, but substandard medications are a rarity because of the checks and balances in the system."