O'Really?

What exactly is a drug? A project I’m currently working on requires a good solid definition, at the very least comprehensible to humans, and preferably understandable by more intelligent, semantically aware computers too. I would like to be able to take some scientific model and ask questions like, “show (or hide) all the drugs in this model”. Trouble is, the word “drug” is such a heavily overloaded term, with many alternative meanings, that it is practically meaningless. Just when you think you have a definition, you can find a case that breaks it. I’m not just being an anally-retentive pedant, well no more than usual anyway. It turns out to be much harder than you might think to define what a drug is. The term drug depends on all kinds of contextual information, dosage, species, legality, intent, social conventions and so on. Here are some broken definitions, warts and all. As you’ll see, the various definitions of drugs don’t work, they just make you worse.

The Oxford English Dictionary (OED subscription required) definition of a drug claims that a drug is:

“An original, simple, medicinal substance, organic or inorganic, whether used by itself in its natural condition or prepared by art, or as an ingredient in a medicine or medicament. Formerly used more widely to include all ingredients used in chemistry, pharmacy, dyeing, and the arts generally, as still in French.”

Let’s ignore the general all ingredients used in chemistry… definition, because it is almost completely useless. Let’s ignore the french too, they aren’t useless, but they’re not helping us much with their vague drogues and chemie. The OED defines a drug in terms of a medicinal substance or medicine, which is

“A substance or preparation used in the treatment of illness; a drug; especially one taken by mouth.”

So we’re stuck with a rather circular definition, a drug is a medicinal substance which is a drug. Shrug. But a drug has got something to do with illness, whatever the hell that is. What about athletes, when they take banned performance-enhancing drugs, that’s not because they are ill, it’s just good old-fashioned cheating. Aren’t they drugs too? Let’s ask the venerable Professor Wikipedia, the wikipedia definition a drug claims that

“A drug is any chemical substance that, when absorbed into the body of a living organism, alters normal bodily function“

Normal bodily function? What the hell is that? Is a headache normal bodily function, or is that abnormal? Is an allergic response a normal bodily function? If I catch a hideous life-threatening disease, will my body be functioning normally?

We could ask the database of Chemical Entities of Biological Interest (ChEBI). Version 45 of ChEBI defines a drug [ChEBI:23888] as

Any substance which when absorbed into a living organism may modify one or more of its functions. The term is generally accepted for a substance taken for a therapeutic purpose, but is also commonly used for abused substances.

Does that make food a drug? When I eat very therapeutic yummy chicken goulash with sour cream biscuits, my bodily function is altered, in a good way. But food isn’t really considered to be therapeutic in that the sense that drugs are. Food isn’t a drug because drugs are non-dietary, not a part of your regular diet. What about nutritional supplements, like vitamin tablets? Are they food or drugs?

ChEBI also states that a drug is-a pharmaceutical (ChEBI:33294), which is:

“Any substance introduced into a living organism with therapeutic or diagnostic purpose.”

So drugs are all about therapy, diagnosis or abuse. Drugs can be administered to diagnose a disease, as well as therapeutic treatment. But when I give an experimental drug to laboratory rats or yeast or E. coli (in the name of Science, of course) is that therapy, diagnosis or abuse? It’s not really any of those. Professor Google (define:drug) has a lot to say on the subject of drugs, most definitions contradict each other, but none mentions species or dosage, which are also significant.

For example, acetylsalicylic acid (ChEBI:15365) in willow trees is just another metabolite that the tree produces, not a drug. But in humans it is a drug, called aspirin, not a metabolite. Likewise, penicillins (ChEBI:17334) when produced by the bacterium Staphylococcus aureus are a defence mechanism, used to kill other species of bacteria. So to S.aureus, penicillin isn’t a drug, to other bacteria penicillin is a lethal toxin but to humans, penicillin is a drug and a very useful one too. Then there is the problem of dose, if I take a couple of paracetamol (ChEBI:46195), it is a drug, if I take a whole bottle, it’s not a drug anymore – but a poisonous toxin that will kill me (see picture, top right).

So it seems we’re back to square one. When we use the word drug, it means just what we choose it to mean – neither more nor less. Trying explaining that to your nearest computer, see if it understands what you mean.

[Rough days picture by Bayat (and Middleman)]

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