Definition of what are you on? in US English:

what are you on?

phrase

informal
  • Said to express incredulity at someone's behavior, with the implication that they must be under the influence of drugs.

    • ‘I stopped and looked at them as they stared and Shannon said, ‘what are you on?’’
    • ‘For god's sake Holly, what are you on?’
    • ‘You are getting creepier and more paranoid than normal… what are you on?’
    • ‘Dude, what are you on? Are you still loaded?’
    • ‘Ma, its four in the morning, what are you on? Did you overdose on painkillers again?’