The antonyms for the word "most nontoxic" include "toxic," "poisonous," "venomous," "dangerous," and "hazardous." While most nontoxic substances are considered safe for humans and the environment, these antonyms describe substances that can be harmful or deadly. Toxic substances can cause illness or death, while poisonous substances can also be deadly when ingested or otherwise exposed to the body. Venomous substances typically refer to those produced by animals, such as snakes or spiders, and can cause severe reactions when injected. Dangerous and hazardous substances are those that pose a risk of harm or injury, either through direct contact or exposure over time.