Why does cannabis smell skunky?

If you've ever taken a whiff of well-preserved, unburned cannabis buds, and thought you smelled something skunky — or, less likely, been on the receiving end of skunk spray and wondered why you reeked of cannabis — you might wonder why the two smell so similar. 

In other words, why does cannabis smell so much like skunk spray? It turns out, the stinky culprits in both substances belong to a family of prenylated volatile sulfur compounds (VSCs), a subset of volatile organic compounds (VOCs), according to a 2021 study published in the journal ACS Omega.

"We suspected that sulfur-containing compounds were the origins of this aroma, just based on chemical intuition as chemists," study lead author Iain Oswald, an analytical chemist at Abstrax Tech, a company that develops aromatic compounds known as terpenes in Tustin, California, told Live Science. After all, "skunks have a bunch of VOCs in their aerosol spray," so perhaps cannabis had similar properties, the team suspected.  

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The sulfurous origins of the plant's pungency reported in the study were perhaps expected, but the novelty of some of the compounds that the team discovered were not, said Amber Wise. Wise is the scientific director and an analytical chemist at Medicine Creek Analytics, a cannabis-testing lab in Fife, Washington, and was not involved with the study.

"The skunky smell, you know, egg smell, if you want to call it 'fart smell' — those are all sulfur compounds, like that's known, at least to chemists," Wise told Live Science. "The surprising part was that there's sort of unique molecules to cannabis." 

Those compounds include VSC6 and VSC7, which Oswald said have not before been found in any plants.

Cannabis has a complicated scent profile, with more than 200 secondary metabolites — small molecules plants produce to defend themselves against predators, such as insects, or to attract pollinators — contributing to its distinctive aroma. Terpenes are chief among those in terms of concentration; about six of the aromatic compounds can each be individually responsible for up to 50% of the variation in odor between different cannabis strains, or cultivars, the authors wrote in the study. For example, the cannabis strain called OG Kush gets its signature gasoline-like smell from the terpenes β-myrcene and β-caryophyllene, while higher concentrations of terpinolene and D-limonene make the cultivar Jack Herer smell like citrus and wood.

While the role of terpenes in individual cultivar profiles has been well established, Oswald and his colleagues — three of whom, including Oswald, have filed a patent related to their findings — wanted to home in on the compounds that give all cannabis strains their skunky odor.

VSCs have relatively complex structures, which have made them difficult to analyze. Moreover, the concentrations of VSCs — which are also found in hops and the pungent fruit durian — in Cannabis plants are minute compared with terpenes.

"The amount actually in the plant is very low, even though it has a very strong effect on the odor itself," Oswald said. "And that's just because the human nose is very sensitive to these types of compounds, so it doesn't take a lot to go a long way."

To understand the structures of the sulfurous compounds, Oswald and his colleagues employed a custom-built two-dimensional gas spectrometry system, combined with mass spectrometry and sulfur chemiluminescence. These techniques allowed them to identify the structures of each VSC, which numbered seven in total. Of those seven, the scientists noted that five contained a functional prenyl group — a feature of VSCs that are found in garlic.

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The scientists then had a trained panel of four members rank the pungency of the diluted VSCs, along with a collection of terpenes and Cannabis flowers, on a scale of zero to 10. The panel noted that VSC3 — which was identified in a 2001 study published in Chemistry — A European Journal as the primary compound in "skunked" beer that has been ruined by exposure to ultraviolet light — most strongly evoked the skunky aroma of cannabis.

"VSC3, by itself, smells like cannabis, essentially at a distance," Oswald said. "The terpenes, by themselves, are very mildly reminiscent."

This was corroborated when the researchers mixed together the top 10 aromatic components of the Bacio Gelato cannabis cultivar but excluded VSC3 — creating a smell that the panel noted was evocative of the strain's floral scent but not its skunky pungency. However, VSC3 doesn't do all of the aromatic heavy lifting on its own.

"There's this synergistic effect," Oswald said. "When we mixed them together, that flavor panel said, 'This is cannabis this; this is definitely it.'"

Oswald also noted that while the 2021 paper mostly highlighted the significant contribution of VSC3 to the cannabis cultivars' funky odor, the compounds VSC4 and VSC5 can also play an important aromatic role.

"VSC3 is by far the most important and correlated the most with the cultivars," Oswald said. "But if we were to ever discover a cannabis cultivator that didn't have VSC3 at all but had VSC4 and VSC5, I promise you, everybody would say that that smells like cannabis."

Originally published on Live Science.