Copied from an old answer on a different sub:

Truthfully, we don't know. We don't even really know which stories of ancient poisoning to believe. Much like modern media, ancient authors attributed mysterious deaths to poison partially out of dramatic effect. On top of that, ancient understanding of disease was poor at best, and many suspected poisonings have very similar symptoms to deadly viruses. Very few ancient sources name specific poisons - again because stories of poisoning are often speculative in the first place. Even fewer mention common poisons in the context of a specific story of a prominent poison victim. Basically, we don't know what any individual Persian poisoner would have used in their given situation, but ancient sources (and modern distribution of poisonous plants) do tell us what poisons they could both access and use.

Knowledge of which plants and substances are toxic is very old. It has always been a crucial survival skill and was almost certainly widespread and understood in pre-history, if not systematically studied. According to legend/tradition passed on by the Hellenistic historian Manetho, the bronze age Pharaoh Menes was the first person to direct a systematic study of poisons and some earlier Egyptian papyri fragments reference poisons and poisonous substances. However, most detailed explanations of poison come from later sources in the Hellenistic and Roman period. Given the widespread nature of the Persian Empire, Mauryan Indian sources are also useful. Livy's History of Rome and other Roman historians occasionally provide references to specific substances, but Pliny the Elder's Natural History is by far the most detailed and useful. Ancient medical treatises like those of Hippocrates and Galen are also useful. In the east, Kautilya's Artashastra and the works of Shushruta both discuss poisons.

Metallic toxins like arsenic and antimony were documented in Egyptian sources and a wide variety of plants were known across the Persian world. From the Mediterranean, opium poppies and mandrake were both cultivated and available. Henbane grew wild in Scythian territory north of the Achaemenid frontiers in Central Asia. Strychthine and Datura both originated in India/South Asia. However, Persians wouldn't even have needed to go that far from home to find perfectly lethal poisons as the infamous belladonna grows natural across West Asia, including Iran.