Poisonous and Toxic

Poisonous describes any substance, natural or manufactured, that is harmful or deadly to living cells, even in small quantities.

Toxic, strictly speaking, refers only to poison that is produced naturally by living things.

In non-technical usage, the word toxic is often used to describe any substance that is harmful or not good for you e.g. ‘toxic fumes’. Although technically incorrect (see above), this is very common usage.