I am increasingly running across people who believe that substances such as caffeine and cannabis are not drugs. Perhaps people hold these ideas because they believe these substances are harmless. Perhaps the idea serves to keep shame or guilt at bay. Some people are able to use some drugs recreationally. The problem is that every drug distorts our perceptions to some degree, often making it hard to see how the drug degrades our quality of life. This explains why people can destroy relationships, flunk out of school, or lose their job while still denying that they have a substance abuse problem.
A commonly accepted definition of a drug is any non-food substance that affects mind or body function.
Here is a neat representation of how common psychoactive drugs are related in their effects.