What category of substances includes those that can cause severe adverse effects when absorbed?

The category of substances that can cause severe adverse effects when absorbed is poison. Poisons are defined as substances that can cause harm to the body when they enter the system, and their effects can range from mild to severe, potentially leading to life-threatening situations. This is primarily due to their chemical compositions, which can disrupt biological processes, damage tissues, or interfere with the functioning of essential systems in the body.

In contrast, medicines are designed to treat or prevent diseases and generally have therapeutic effects when used correctly. Vitamins, while important for health, are nutrients that typically support bodily functions and can lead to negative effects only in extreme doses. Allergens provoke immune responses, which can lead to adverse effects primarily in susceptible individuals, but they do not typically cause systemic harm in the same way that poisons do.