"The dye test will assist national malaria control authorities in urgently needed campaigns to stop this murderous trade.
Researchers examined drugs to treat malaria and tuberculosis and some antibiotics.
Many were simply fakes of the real thing, while others had deteriorated because they had been kept in poor conditions.
Out of 104 samples, 38% contained no artesunate at all, despite the packaging stating that they did.
The researchers said this had definitely caused malarial patients to die unnecessarily. But they said the use of a simple dye would have highlighted which drugs were the genuine article.
"The illicit trade in counterfeit malarials is a great threat to the lives of people with malaria.
"The dye test will assist national malaria control authorities in urgently needed campaigns to stop this murderous trade.
"The recent emergence of counterfeit artesunate in this region has led to the death of many patients who would have survived if given the genuine drug."
Climate problems
In a separate paper in The Lancet, researchers from the Robert Gordon University, Aberdeen, studied anti-malarial, anti-bacterial and anti-tuberculosis drugs from Nigeria.
They found that out of 581 samples from 35 random pharmacies, almost half fell below international standards and either had too much or too little of the active ingredients.
Part of the reason for the poor quality of drugs was due to the decomposition of the active ingredient because of high temperatures and humidity of storage.
The preparations came from 12 different countries, but 38 failed to carry a label showing the country of origin.
The researchers concluded that the sub-standard drugs could contribute to drug resistance and the failure to combat infectious diseases.
They said: "Where the amount of active drug is well below stated amounts, use of these preparations could lead to therapeutic failure."
Legal moves
Professor Li Wan Po, from the Centre for Evidence-Based Pharmacotherapy, at Aston University, Birmingham, said the prospects of combating the problem looked bleak.
But he said the full force of the law should be brought down on those profiting from drug counterfeiting.
And he called for local training programmes to help people spot potential problems with the drugs they are selling.
"The enforcement divisions of local drug-regulatory agencies should be strengthened and, if necessary, officials and directors changed in order to abolish fraud.
"Offenders should be barred from trading and those responsible brought to the attention of the public and the law.
"Although optimism is hard to come by in this area there is hope that the combination of these efforts may yield some success."