If you accept an assignment to a particular job that requires one. They surprise you with it. You have to submit to the drug test right then and there. They won't tell you that they require it till you are in their office, even when they call you to let you know that you have a job! Sneaky.
Once the job offers you a position
Answered - Laborer (Former Employee) - Fairfield, IA
Before you sign the employment paperwork for each job they want to hire you for.
Answered - Machine Operator (Former Employee) - Elkhorn, WI
Answered - Specialist, Product Logistics (Current Employee) - Research Triangle Park, NC
Never had one
Answered - Inspector (Current Employee) - West Bend, WI
When you get the job
Answered - Packaging (Former Employee) - Cedar Rapids, IA