【Poisonous】 ,  【venomous】 ,  【virulent】 ,  【toxic】 ,  【mephitic】 ,  【pestilent】 ,  【pestilential】 ,  【miasmic】 ,  【miasmatic】 ,  【miasmal】  are comparable when they mean having the properties or the effects of poison (see POISON )

Basically  【poisonous】 implies that the thing so described will be fatal or exceedingly harmful if introduced into a living organism in sufficient quantities (as by eating, drinking, or inhaling). In its extended use the term implies extreme noxiousness or perniciousness or power to corrode, rankle, or corrupt.

【Venomous】  applies equally to an animal (as a snake, scorpion, or bee) whose bite or sting introduces a venom (see venom under POISON ) into an organism and to the bites, stings, or wounds inflicted by 【venomous】 creatures. The term has much extended usage; in this it implies extreme malevolence or destructive malignancy.

【Virulent】 implies the destructive or extremely deleterious properties of or as if of a strong poison; it is applied especially to infectious diseases of a particularly malignant or violent form or, somewhat less often, to notably 【venomous】 animals. In extended use the term applies to something particularly violent in its display of an offensive or noxious nature or quality.

【Toxic】  sometimes implies the presence of properties or effects of a toxin (see toxin under POISON ), but more often implies only the character or the properties of a poison and therefore means little more than  【poisonous】 .  In its extended use  【toxic】 may imply insidious and destructive activity comparable to that of some toxins in the human organism.

【Mephitic】 is applicable to something so offensive to the sense of smell that it is or is believed to be actually 【poisonous】 .

【Pestilent】  and  【pestilential】  occasionally come close to  【poisonous】 in meaning, but they are chiefly used in the extended sense of exceedingly infectious or dangerous to the health, morals, or mental integrity, especially of the group as distinguished from the individual.

【Miasmic】 ,  【miasmatic】 , and  【miasmal】  all imply a reference to miasma, or supposedly infectious or deadly emanations from swamps or jungles or from putrescent substances that float in the air.

Of these words only  【miasmic】  is common in extended use, where it often comes close to  【pestilential】 in implying a power to spread contamination or to poison the minds or souls of the multitude.