Introduction

A hemp extract is often summed up in a single number: its CBD percentage. That’s convenient on a label, but pharmacology research tells a far more nuanced story. A scientific review published on 13 August 2026 in Frontiers in Neuroscience surveys hundreds of studies and reaches a conclusion that should interest anyone curious about hemp: what sets two preparations apart is almost never a single molecule.

The amount administered, the way it enters the body, the duration over which it is repeated, and above all the proportions between the compounds present — these are the four variables the authors identify as decisive. Terpenes, long regarded as simple aromatic molecules, occupy a surprisingly central place among them.

This article summarises what this review says about extract composition. It is pharmacological research, conducted predominantly on animal models: nothing that follows constitutes information about product use.

A review published in Frontiers in Neuroscience

The work is authored by Kaylin J. Ellioff, Madalyn Critz, Nephi Stella, Michael R. Bruchas and Benjamin B. Land, from the Department of Pharmacology at the University of Washington in Seattle. It was published on 13 August 2026 in the Neuropharmacology section of Frontiers in Neuroscience, in open access, under DOI 10.3389/fnins.2026.1888863.

This is not a single experiment but a literature review: the authors gather and compare the findings of a large number of previous studies — both preclinical and clinical — to identify what is well established and what is not. Their field of study is the pharmacology of analgesia, a medical research discipline; this article draws only on the section devoted to compositional determinants.

Their opening observation is clear: published results on cannabis have historically been highly heterogeneous. The authors argue that this dispersion does not reflect disorder in the research, but the fact that too many variables are acting simultaneously and that few studies control for all of them.

Four variables that change everything

Dose

THC illustrates the problem clearly. The authors describe it as a molecule whose effects are reliable and reproducible in animal models, but whose window is narrow: the amounts at which the target effect appears overlap with those that produce the unwanted effects linked to the CB1 receptor — sedation, drop in body temperature, increased appetite, and impaired motor function.

In other words, simply increasing the dose is not enough. Beyond a certain threshold, what grows is not the intended effect but the cluster of drawbacks that comes with it.

Route of administration

This is one of the most striking points in the review. The same molecule, administered by a different route, can produce opposite responses.

The authors cite the case of limonene — the citrus terpene found in many hemp varieties. When injected locally, it triggers a response via a cellular channel called TRPA1. When administered systemically, it dampens responses that depend on that very same channel. The route of entry determines the direction of the effect here, not just its intensity.

In the same way, a THC-CBD combination administered orally does not produce the same results as the same combination administered subcutaneously or at the spinal level.

Duration

Repetition is not the same as accumulation. THC administered repeatedly leads to tolerance — the body responds less and less to the same amount — and to dependence.

CBD follows the opposite pattern. The authors describe limited and inconsistent acute effects after a single dose, but more consistent results when administration is repeated over time, particularly in models of neuropathic pain and chemotherapy-induced neuropathy. The two molecules therefore behave very differently over time.

Composition

This is the thread running through the entire review. With dose and route held equal, it is the proportions between the compounds present that shape the measured response. And the result is almost never a simple addition.

Terpenes are more than aromatic molecules

The review dedicates a long section to the best-documented terpenes: linalool (dominant in lavender), beta-caryophyllene, myrcene (found in hops and mango), limonene, alpha-terpineol and alpha-bisabolol. These are also, according to analyses of commercial products cited by the authors, the most abundant terpenes in hemp available on the market.

The key point: these molecules have pharmacological targets of their own, distinct from those of cannabinoids. The studies surveyed describe activity on adenosinergic signalling at the spinal level, on the CB2 receptor expressed by immune cells, and on endogenous opioid pathways. Myrcene is a particularly interesting case: laboratory tests show that it does not directly activate the CB1 receptor, yet its effects disappear when that receptor is blocked — suggesting an indirect action, modifying the activity of the endocannabinoid system rather than substituting for it.

The authors’ conclusion on this point is worth quoting directly: machine learning data analyses suggest that terpene composition predicts the observed response better than THC or CBD content alone.

For an extraction laboratory, that is a significant statement. It means that a displayed percentage, on its own, gives a very poor description of what an extract actually contains.

The entourage effect: a double-edged mechanism

The “entourage effect” refers to the idea that the compounds in a plant interact with one another and together produce something other than the sum of their individual actions. The review confirms that the phenomenon is real — and notes that it does not systematically work in the hoped-for direction.

The authors report situations where a low-total-dose THC-CBD combination produced a response clearly greater than what simple addition would predict, without cannabinoid-type side effects. But also the reverse: in another protocol, an oral combination in equal parts produced no synergy on the target effect, while generating a synergy of side effects that grew as the proportion of CBD increased.

Interactions also depend on more than just proportions. The timing and route of co-administration matter: the same compound given before another can alter its metabolism and blood concentration, whereas the same compound given differently changes nothing. The authors also flag differences linked to the sex of the animals, with certain ratios producing responses in males but not in females.

The lesson is clear and runs counter to a common assumption: adding more molecules to an extract does not automatically improve its profile. It is balance that decides.

Why research on hemp moves slowly

The final section of the review addresses a methodological problem. How can you tell, in an animal, whether a substance has genuinely altered a response — or has simply made the animal drowsy and therefore less reactive?

The authors propose a technological answer: computer vision. Algorithms analyse the animal’s posture and movements frame by frame, identifying movement signatures that a human observer cannot distinguish with the naked eye. This makes it possible to analytically separate a specific response from simple sedation or a reduction in activity.

This is a methodological detail, but it explains a great deal: part of the heterogeneity in results published to date stems from the difficulty of cleanly measuring what one believes one is measuring.

Conclusion

The review published in August 2026 by the University of Washington team delivers a consistent overall message: the response observed after administration of hemp compounds depends on dose, route of administration, repetition over time and — above all — the proportions between the molecules present. THC emerges as a narrow-window molecule, CBD as a molecule whose profile takes shape over time, and terpenes as full pharmacological actors rather than mere fragrance.

The authors themselves set out the limits of the exercise. This is a narrative review rather than a systematic meta-analysis; the bulk of the data discussed is preclinical, obtained in rodents, with all the caution that imposes before any transposition; and the field remains, in their own words, one of the least well resolved in terms of mechanisms.

One lesson runs through the entire review and connects directly to the work of an extraction laboratory: a hemp extract is not the sum of its components. The balance between the cannabinoids present — CBD, CBG, CBC — and their environment of terpenes and flavonoids is a defining characteristic of the finished product, just as much as its stated concentration. Two oils showing the same CBD percentage can rest on very different phytochemical profiles, and behave very differently under analysis.

This is precisely why we pay such close attention to the composition of our extracts and to the processes that shape them. Fractional distillation, used for the majority of our