A toxin (or poison) describes any substance that poses an immediate threat to human life or health by interfering with a necessary chemical reaction in the body, but otherwise has no therapeutic use.
It should be noted that most substances are toxic when they exceed a certain dose or concentration, even oxygen. Most pharmaceuticals are toxic beyond their therapeutic dose, and some (such as the chemicals used in chemotherapy) are toxic even at their therapeutic dose.
Most toxins interfere with bodily functions at the molecular level. For example, most nerve toxins block the uptake of the chemicals that are released at the end of nerve cells. As all toxins have a different effect, each requires a specific treatment, often known as an antidote. For example, each type of snake venom must be treated with an anti-venom tailored to that particular species.