Thalidomide was used by some women to relieve morning sickness
Scientists claim they have finally worked out how and why the drug thalidomide caused limb defects in thousands of babies.
Mothers had taken the drug to relieve their morning sickness in the late 1950s and early 1960s.
A University of Aberdeen-led team said a component of the drug prevents the growth of new blood vessels in the developing embryo.
About 10,000 babies whose mothers took the drug were born with disabilities.
The most common defect was to the limbs of babies. They were born with stunted arms or legs or, in some cases, no limbs at all.
We have put to rest a 50-year puzzle, in finally deducing how thalidomide triggers limb defects and why it appears to target limbs preferentially
Dr Neil Vargesson University of Aberdeen
It took some time for evidence of birth defects to be linked to thalidomide.
In the 1960s, it was discovered the drug had an effect on some of the painful symptoms of leprosy.
And in the 1980s, scientists once again became interested in the drug's complex properties when researchers began to explore its use in the treatment of a number of diseases, such as cancer.
The Aberdeen researchers said the drug could be re-developed without the component they have highlighted.
Lead researcher Dr Neil Vargesson said: "We have put to rest a 50-year puzzle, in finally deducing how thalidomide triggers limb defects and why it appears to target limbs preferentially.
"Thalidomide is a complicated drug with many actions, but which of these cause limb defects and, as importantly, how this action causes defects, has eluded researchers for many years.
"Many theories have been put forward but this is the first paper to conclusively show that it is the antiangiogenic property of the drug - that element that inhibits new blood vessel formation - that is to blame for the defects."
He said the drug was taken by women early in their pregnancy to combat morning sickness, typically at around five to nine weeks, although morning sickness in some women can last a lot longer.
"This specific timeframe is crucial as that is when the limbs of babies are still forming," he said.
"The blood vessels involved in this process, at this stage of pregnancy, are still at an immature stage when they rapidly change and expand to accommodate the outgrowing limb."
Dr Vargesson added: "It remains possible that we could make a safer form of the drug that has the clinical benefits for sufferers of leprosy but does not cause limb defects."
However, Dr Martin Johnson of the Thalidomide Trust criticised the research.
He said: "The study does not appear to take into account the wide range of damage caused by thalidomide other than limb damage.
"It also does not take into account explanations that are already widely accepted in the scientific community."
The paper has been published in Proceedings of the National Academy of Sciences.
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