You need consent regardless of age and a responsible adult will be required under 18.

You will need to clearly communicate the company policy so that informed consent can be made.

Dropping a test on someone the day they step through the door could land you in a lot of issues so it would be wise to include the requirement in communications before the event.

Companies could save themselves wasted time if they include comment about mandatory D&A testing during their recruitment process.

Hi Alicja

What are you describing as "safety critical" tasks?

These two words mean all sorts of things to different people.

So, there is work where legislation says that drug and alcohol testing is a legal requirement for those doing "safety critical" work - usually where an incident could be easily foreseen to result in multiple casualties, e.g. a rail crash.

Those with experience in the relevant sectors should know the rules and have recruitment policies in place to avoid the sorts of issues that Roundtuit rightly points to. 

Then there is work which an organisation decides to CALL "safety critical" - what a person do could result in one (even two) deaths.

AND there are the organisations that decide to extend D&A testing to a much broader section of their workforce than those actually doing what is deemed by legislation as "safety critical".

As soon as you go beyond those within the scope of the relevant legislation, you should be thinking very carefully and probably involving HR and perhaps the lawyers.