How can you define a “drug”? Nobody really knows

What is a medical drug? Ask anyone on the street, and they’ll probably tell you it’s the kind of thing you pick up when you’re not feeling well.

This understanding is wrong, as we will see. But after a thorough investigation, my colleagues and I found no other potential definition is better.

Despite their centrality to medicine, we have no idea what medical drugs are. We can’t even tell the difference between drugs and food, let alone drugs and so-called “natural” alternatives.

A definition of Goldilocks

In a recent article in the Journal of Medicine and Philosophy, my colleagues (Sara Linton, pharmacist, and Maureen O’Malley, philosopher of biology) and I tried to nail down a viable definition of medical drugs.

A viable definition must be broad enough to include anything classified as a drug. To get a sense of this “everything”, we used the drug bank compiled by the Canadian Institutes for Health Research, which lists more than 16,000 substances.

A definition must also be narrow enough to exclude substances that are not generally considered drugs. Take food, for example. Eating a sandwich is never thought of as taking a drug.

In short, a viable definition of what drugs are must occupy a “Goldilocks” zone between these two questions: big enough to include all drugs, small enough to exclude everything else.

Based on an initial study of pharmacology books, we found three broad ways to define drugs: in terms of what they are, how they work and what they are used for.

Unfortunately, none of these options are in the Goldilocks zone.

Are they specific chemical drugs?

If all drugs were a particular type of chemical, then defining drugs would be easy. But this idea is hopeless: there is nothing, chemically speaking, all drugs have in common.

It is also tempting to think that drugs are “artificial” chemicals, made in a laboratory, while “natural” supplements come from nature, and that is the difference.

But many drugs are “natural” in this sense. Aspirin, for example, is derived from willow bark.

Read more: Modern medicine has its scientific roots in the Middle Ages – how the logic of vulture brain and bloodletting remedies lives on today.

This has immediate implications for so-called “natural” supplements, such as fish oil.

If “drugs” are chemically indistinguishable from “natural” supplements, the supplements should not be considered a “safe” alternative. Supplements are no less, and no more, safe than many drugs.

Do drugs perform a specific function?

Perhaps drugs can be defined in terms of what they do. This idea initially seems promising, as many drugs work by binding to receptor molecules in the body.

Think of a lock and key: the receptor molecule is the lock, and the drug is the key that opens it.

The discovery of receptor molecules is significant. For some, it is the “big idea” of the science of pharmacology.

But this definition of medical drugs is also hopeless. Many drugs do not bind to receptors. Antacids, for example, only work to change the level of acidity (pH) in a person’s body.

Read more: Explainer: what is the placebo effect and are doctors allowed to prescribe it?

Many placebos also bind to receptors. Placebos are often contrasted with drugs, but the definition of drugs as “things that bind to receptor molecules” includes many placebos in the definition. So this definition doesn’t work either.

Is there a way to define placebos that clearly distinguishes them from drugs? This is not obvious, since the definition of placebo is still very difficult.

For example, one might think that placebos are substances that have no therapeutic effects. But placebos can have therapeutic effects (the so-called placebo effect), so this definition does not work. A number of other definitions face similar problems.

Our research paves the way towards an explanation of why it is so difficult to define placebos. To properly define placebos, we need to differentiate them from drugs, which we cannot do without a definition of what drugs are.

The drug makes me better

This brings us back to wellness. In this view, a medical drug is simply any chemical substance used in medical treatment.

This is better: capturing the full range of substances used as drugs in medical contexts.

But now there is absolutely no hope of keeping food and nutrients out.

Read more: Do you care? Traditional Chinese medicine shows that context can make all the difference

Consider, for example, total parenteral nutrition. This is a chemical infusion given to people who have trouble absorbing nutrients normally.

Total parenteral nutrition is used in medical treatment. But what it does for your body is really no different than what a good sandwich does.

Any account based on drug treatment will inevitably remove the contrast with food.

So what?

In our daily lives, we make choices that rely on an implicit understanding of what drugs are.

For example, we take paracetamol because it is a drug. Many of us may still take fish oil precisely because we believe that it is not a drug.

Without an account of what drugs are, we risk making serious mistakes.

We may take substances that we think are “inert” (placebos) because they are “natural” (like fish oil) when in fact they are active drugs.

Likewise, all legal regulation of medical drugs assumes that we already know what a drug is.

But we don’t: our understanding is clearly evolving. This means that regulation must also change continuously. Substantial resources must therefore be devoted to reforming legislation as we continue to rethink what medical drugs are, as the recent reclassification of MDMA and psilocybin as medicines in Australia demonstrates.

Then there is food. Food is not administered or regulated as a drug in a hospital, with the exception of total parenteral nutrition and similar substances.

But if doctors use food as a drug to contribute to the well-being of patients, then perhaps they should be subject to the same standards.

This may require a radical rethinking of the way meals are provided in a hospital. Perhaps meals should be administered, and regulated, with the same care as drugs.

Hospital meals could never be the same. But that could be a good thing.

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