Myrcene, And How To Read The Largest Number On A Profile

A hazard identification asks a single question: under some set of conditions, can this substance cause a particular kind of harm at all? It is a yes or no about capability.

A risk assessment asks a different question: given realistic exposure — this much, by this route, for this long, in this species — how likely is that harm?

The two get conflated constantly, and the conflation runs in both directions. "Listed as a carcinogen" is read as "dangerous at any dose", which does not follow. "Present in food" is read as "therefore harmless in any form", which does not follow either.

Water, sunlight and salt all have dose-response curves with a harmful end. The question is never whether a substance can harm. It is always at what exposure.

The United States National Toxicology Program administered beta-myrcene of greater than 90% purity by gavage — a tube directly into the stomach — to F344/N rats and B6C3F1 mice, for three months and for two years. The results are published as Technical Report 557.

Under the conditions of the two-year studies, the programme reported clear evidence of carcinogenic activity in male rats, based on increased incidences of renal tubule neoplasms; equivocal evidence in female rats; and clear evidence in male mice, based on increased incidences of hepatocellular adenoma, hepatocellular carcinoma and hepatoblastoma. In the three-month study, renal tubule necrosis was increased in all dosed groups of both sexes. The programme's own assays did not find evidence of genotoxicity.

Those phrases are terms of art with defined meanings in that reporting system, and they describe the study rather than the world. Read the conditions alongside them: a purified compound, delivered daily by tube, at doses measured in tens to hundreds of milligrams per kilogram of body weight, for the animal's entire adult life.

California's Office of Environmental Health Hazard Assessment listed beta-myrcene under Proposition 65 as known to the state to cause cancer, effective 27 March 2015, through what is called the authoritative bodies mechanism.

That mechanism matters for interpretation. It means the listing followed automatically from the National Toxicology Program's formal identification, rather than from an independent state review of realistic exposure. Proposition 65 listings are hazard identifications by design. They are not statements that any particular exposure is unsafe, and the statute does not ask them to be.

The same compound has separately been at the centre of a long argument in United States food law about the Delaney Clause, which bars approval of a food additive found to induce cancer in animals irrespective of dose. Irrespective of dose is the entire point of contention.

It would be convenient if the rodent findings could simply be set aside as not applicable to people. They cannot, entirely, and the honest account is more interesting than either comfortable version.

A 2013 histopathology analysis of the kidneys from that bioassay described an unusual dose-related nephrosis alongside low-dose alpha-2u-globulin nephropathy. Alpha-2u-globulin nephropathy is a mechanism specific to male rats — the protein involved has no human counterpart of consequence — and findings attributable to it are generally not carried across to human risk assessment.

So part of the renal finding has a recognised mechanistic reason to be discounted for humans. Part of it, at the higher doses and in both sexes, was attributed to the nephrosis rather than to that mechanism, and does not get the same exemption. The liver findings in mice sit outside the argument altogether.

The result is a genuinely mixed picture that neither a reassuring summary nor an alarming one represents fairly.

Set the listing aside for a moment. Myrcene is not an obscure compound, and the pharmacology on it is more specific than its reputation suggests in either direction.

The antinociception work from around 1990 is the most-cited, and the mechanistic detail in it is the interesting part rather than the headline. Rao and colleagues reported inhibition of nociception in mice, and then established that the effect was abolished by naloxone and by yohimbine — an opioid antagonist and an alpha-2 adrenoceptor antagonist respectively. That pairing is not what a non-specific sedative effect looks like, and it led the authors to propose alpha-2 adrenoceptor stimulated release of endogenous opioids rather than any direct action at an opioid site. Whether that mechanism operates at exposures a person encounters is a separate question the study did not set out to answer. As pharmacology, though, it is a specific and falsifiable proposal rather than an observation, which is more than most of this literature offers.

Its physical chemistry matters too, in a way that bears directly on the certificate. Myrcene is a small, highly lipophilic hydrocarbon with no oxygen in the molecule. That makes it volatile, poorly water-soluble and readily partitioned into lipid membranes — properties which shape how fast it leaves a container, how it distributes in tissue, and why the compound most likely to top a fresh profile is also among the first to fall away from it.

Neither of those is a statement about what the compound does for anyone. They are the reasons it is worth studying at all, and the reason a large myrcene figure carries real information about a material's freshness and volatility even where it carries none about outcome.

A figure on a certificate of analysis is a composition measurement. It reports how much of a compound was in one sample on one day.

It is not a safety verdict. The presence of a compound with a hazard listing is not evidence that a given exposure is harmful, and the absence of a listing is not evidence that a compound is inert — most substances have simply never been through a two-year bioassay, which is expensive and slow.

It is not an effect prediction either. Myrcene carries a popular reputation built largely on rodent studies in which the compound was injected at tens of milligrams per kilogram, which describes those experiments and not a person.

The same molecule therefore arrives with a folk reputation from one rodent literature and a regulatory listing from another. Neither of those is what the number on the page is telling you. The number is telling you what was in the sample.

When a listing or a study is put in front of you, four questions settle most of it.

What was the dose, expressed per kilogram of body weight, and how does it compare with realistic exposure? What was the route — swallowed, inhaled, injected, applied — and does it match the exposure in question? What species, and is the mechanism one that species has and humans do not? And is this a hazard identification or a risk assessment, because the two answer different questions and are not interchangeable.

Most frightening chemistry headlines and most reassuring ones fail at least one of those questions. Asking them is not scepticism about regulation; it is how regulators read their own documents, and it is the standard this practice applies when working out what a certificate is actually saying.