When my grandparents sleep, I'm going to use their drugs.
Why do you want to take their drugs?
They think I'm a good kid and they will never realize they are missing.
Even if you don't get caught, this is still highly irresponsible. You shouldn't do any kind of drugs if they weren't prescribed to you. You don't know how it will affect you and your grandparents might really need that same medicine.
I don't give a fuck, they think I'm a good kid.
Again, the fact that they won't suspect you is not the issue here. I hope you realize that taking drugs without a prescription could cause you serious health effects, and it could almost certainly affect your grandparents if they run out early. At least consider the practical consequences before you do such a thing.