How to dress for drug test?

Hi this may sound kinda silly to ask but does it matter what I wear to a drug test?

This is for a pre-employment onboarding process and it’ll be on site as I’ll be working in a clinical environment.

I will also be guided by a hiring “expert” (what the manager called them) so I’m not sure if I should dress like how I did for my interview or if I can go more casual…