The raid took place at St David's 2 shopping centre in Cardiff and specially trained sniffer dogs were used to carry out the search on more than 500 workers.
The main contractor, Bovis Lend Lease, reported a number of workers failed to turn up for work on the day of the screening which raised suspicions that they had been told beforehand by other members of the workforce.
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The results revealed that a 12 employees had either used drugs or been in close contact with drugs.
No drugs were found and none of the workers have been prosecuted.
Nick Wylie, Project Director For Bovis Lend Lease said: "We will be doing other random tests in the future along the same lines. I would commend other contractors to do the same because unless we are seen to do something, drug abuse will continue unabated."
(CD/JM)
