The shakespearean character juliet took a potion that paralyzed her and affected her muscles used for breathing. she probably took something that blocked the action of

She probably took something that blocked the action of acetylcholine.

Acetylcholine is a chemical whose function is to transmit nerve impulses from a cell to another cell within central and peripheral nervous systems. Given that this chemical has been blocked when Juliet drank the potion, she won't be able to breathe which is the reason why she died.