Well, that drug exists, it's called standing up with good posture.
Not only will people treat you with more respect, your body will swirl with a mild ambient pride.
Just try it and see how it affects your mood. Your muscles are probably too weak to stand up straight reliably now, but your back will quickly strengthen. It only takes a month.
So how should I fix my posture?
Here's an Eigenrobot quote that helped me: "Stand like you have hooks through your nipples on lines up to heaven and God is keeping on the tension." But more seriously, whenever you walk somewhere remember to do it once until you are tired. It will get easier.
The military insists on all soldiers standing up straight. And now I understand why they are so damn insistent about it.