Helped me though so thanks to all who have answered me.I'll try and explain a bit. If they've arrested him for a drug driving offence then he's tested positive on a drug swipe.
He will have had a blood test to confirm these levels. That will take months to come back. If he's under on that they can't charge for it.
They'll always go for the 'highest' charge so one will outrank others if that makes sense. This could mean that some charges are dropped.