According to Webster's Medical Desk Dictionary (which I happen to have on my desk, and is partly US-centric):
drug n 1a a substance used as a medication or in the preparation of medication b according to the Food, Drug and Cosmetic Act(1): a substance recognized in an official pharmacopoeia or formulary (2): a substance intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease (3): a substance other than food intended to affect the structure or function of the body (4): a substance intended for use as a component of a medicine but not a device or component, part, or accessory of a device 2: something that causes addiction or habituation
(Note 3: a substance other than food...)
In my biologist's view, I think of a drug as a compound not produced by the body itself which nonetheless can interact with the cells of the body. In other words, cells have these receptors for their own reasons, and it turns out that chemicals from other sources can stimulate (or block) these receptors. (Note, a ligand binds a receptor and inhibits or stimulates the cell by a cascade of subsequent biochemical effects. This is a special field of interest for me.)
For example, we make "endogenous opioids", which are peptide (protein fragment) molecules. These are released in response to pain and stress (runner's high, or jalapeno-lover's buzz). The opioid drugs long known in human culture bear no chemical resemblance to the version the body makes, yet they stimulate the specific opioid receptors pretty well -- sometimes better than the native ligand. Similarly plant cannabinoids look nothing like the arachidonate-related natural ligands.
By these definitions, ephedra and St. John's Wort contain drugs.
I like herbal supplements. Echinacea usually works for me if I take it at the first sign of a cold. However, I'm acutely aware that the active ingredients are technically drugs. A tea of willow bark contains salicylic acid, the same stuff in a pill of aspirin. Chewing coca leaves puts cocaine in your system. Willow and coca could be called herbal supplements or "food," but they contain drugs.
Not all drugs cause addiction, and not all people will become addicted to typically addictive drugs. The receptors for drugs vary among people, as do the wiring/biochemical systems underlying addiction in the brain. The social issues surrounding the currently illegal drugs are as complicated as the physiological processes those drugs affect.
Definitions are easy. Answers aren't.
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