It might be, but I’m not sure what “it” refers to. Whether something is poisonous depends on the substance, the amount, and how it gets into the body—by swallowing, breathing it in, or getting it on the skin.
What substance, plant, animal, or product are you asking about? If someone may have been exposed to something harmful, contact your local poison-control center or emergency services right away. Don’t make them vomit unless a medical professional tells you to.