They added that the problems will get worse as a result of the influx of young workers and the prevailing culture of drug use.
Problems raised as a result of intoxication at work include lethargy and over-exuberance, both of which are potentially fatal on site.
One graduate trainee civil engineer said that he has had to turn workers who were still intoxicated from the night before away from sites as they were unsafe to operate machinery.
Two major sites at which reports of drug use have been made are the £250m SmithKline Beecham headquarters in west London and the £78m Millennium Wheel in Falkirk, Scotland.
Investigations were launched into both claims, but no evidence to substantiate them was found.
The problem was highlighted in 1999, after it was alleged in the national press that ecstasy and cannabis were being dealt during the construction of the Millennium Dome.