I understand drug abuse on construction sites is not a new but continious issue for construction sites all over the country,  and people can refuse to be drug tested. But what is the best way to tackle this issue, from a health and safety perspective to keep people safe?

For someone with a lot of construction experience you seem to have forgotten it is wholly within the control of the prinipal contractor to set site rules which override any subcontractor employment contract.

Ergo if as principal I have set a policy stating drug testing will occur for everyone at site and specifically include refusal to be tested as an exclusion factor then in theory I should have a clean site.

Reality then kicks in with available testing, its tolerances and margins for error and chosen limits.

Roundtuit, to be fair, Karan pointed out a few postings back that they are seeking an in to construction site experience, so not claiming to be experienced in the sector.

Karan, what would you do if this wasn't construction? You mentioned you worked for Heinz. Very much doubt that nobody who ever worked for Heinz didn't dabble in drugs (by whatever definition).