a. Something that has been purified.

b. Something that causes a characteristic change in an organism.

or c. Something that is chosen because it might cause a characteristic change.

---

Once a drug has been identified, there are some questions to ask:

• What are its side effects?
• How small of an amount can have an effect?
• Does it interact with anything else?
• Does chronic use of it cause problems? At what levels? What problems?
• Does its usage affect other organisms?
• What do you need to do to stop using it safely?
• Can it cause acute harmful effects? At what levels? What effects?

A key point of this definition is that it does not specify that a drug must be a substance. Also a drug is defined by the change, not by its design.

I’ll come back to this definition once in a while to show how it can be a useful frame to think about important issues.