Toxicology tests have shown that two teenagers whose deaths were linked to mephedrone had not taken the drug.
The deaths of Louis Wainwright, 18, and Nicholas Smith, 19, in March 2010, sparked concern about the synthetic stimulant, which was then legal.
The Labour government banned the drug in April, making it a Class B drug alongside amphetamines and cannabis.
But tests have revealed there were no traces of mephedrone in their blood.
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