In the final act of Romeo and Juliet, our tragic heroine takes a potion to fake her own death and place her into a catatonic state. Many believe the potion is most likely to be deadly nightshade (Atropa Belladonna) a plant native to Europe.
Other soporific candidates that are found in nature could have been the seeds of the bulrush plant, or a herb called leopards bane. Both of these plants are toxic, however none would have the capacity to induce a coma with a heartbeat so slow it could be mistaken for death.
Upon finding Juliet and believing her to be dead, Romeo uses a powerful, fast-acting poison to take his own life. An obvious choice for such a strong poison is potassium cyanide or the medieval monkshood, both of which cause rapid respiratory failure.