Key difference: A poison is any substance that is harmful or deadly to someone, essentially to living cells. It may be natural or manufactured. A toxin, on the other hand, is a poison that is produced naturally or biologically by living things. Hence, it can be said that toxins are a sub-category of poisons.

Sometimes a substance is considered toxic, someone else might say that no, it is poisonous, while another will say that it is both toxic and poisonous. So, which is it, toxic or poisonous? And, if the words are synonyms, then how can it be toxic and poisonous, wouldn’t that just be redundant? Actually, they are synonyms in the lay man’s terms. However, there is a slight difference in each one’s connotation.

According to Dictionary.com, a poison is “a substance with an inherent property that tends to destroy life or impair health.” This is a vast definition and can include numerous substances.

Dictionary.com defines a toxin as, “any poison produced by an organism, characterized by antigenicity in certain animals and high molecular weight, and including the bacterial toxins that are the causative agents of tetanus, diphtheria, etc., and such plant and animal toxins as ricin and snake venom.”

Effectively, a poison is any substance that is harmful or deadly to someone, essentially to living cells. It may be natural or manufactured. A toxin, on the other hand, is a poison that is produced naturally or biologically by living things. Hence, it can be said that toxins are a sub-category of poisons.

Toxins can be small molecules, peptides, or proteins that are capable of causing disease or harm. A person may come in contact with a toxin by touching and absorption through skin and by body tissues (poison ivy, poison oak or a bee sting), or by ingesting (eating a poisonous plant). A poison is essentially any harmful substance that is created by man, usually chemically. A poison is usually ingested (eating rat poison or drinking bleach) or injected (euthanasia).

Another difference between a toxin and a poison is in the fact that a poison is usually deadly even in small quantities. However, the toxicity of substances varies. One substance may be mildly toxic, resulting in rashes or allergies, while another may be deadly in even the smallest quantity. Most toxins, however, are harmful in small quantities, but could be life-threatening in medium to large quantities.

Sometimes the terms overlap, especially in the case when a naturally occurring toxin is derived by humans chemically for use. 

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