What term describes a chemical substance that causes changes in mind and body functions?

The correct term for a chemical substance that causes changes in mind and body functions is "drug." In the context of health and medicine, a drug refers to any substance that, when introduced into the body, can alter its physiological functioning. This encompasses a wide variety of substances, including medications designed for therapeutic purposes as well as recreational drugs that may affect mental states or bodily functions in different ways.

For instance, medications are a specific type of drug used to treat or manage conditions but do not encompass all types of substances that can alter functions. A toxin, on the other hand, refers specifically to harmful substances that can cause illness or injury. The term "chemical" is more general and refers to any pure substance made up of chemical elements, not necessarily implying any effect on bodily or mental functions. Thus, "drug" is the most accurate term to describe a substance that influences mind and body functions.