Dr. Ethan Russo Endorses 1% THC Limit for Hemp

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World-renowned neurologist and cannabis and endocannabinoid scientist explains why increasing hemp’s allowable THC content makes sense.

January 27, 2021

As the U.S. Department of Agriculture (USDA) works toward a final rule to regulate the legal U.S. hemp market, one point of contention among industry constituents stands above all others: the total allowable tetrahydrocannabinol (THC) content in legal hemp. The Agriculture Improvement Act of 2018 (the 2018 Farm Bill), which federally legalized hemp, set the THC limit at 0.3%.
dr ethan russo

It is clear that legislators are concerned with THC’s intoxicating effects and therefore set a low THC limit for hemp. But the researchers who established that limit never intended for it to be used as a policing measure.

Canadian researchers proposed that limit in their 1976 study classifying cannabis varieties to differentiate their THC-rich plants from their hemp crops. “We arbitrarily adopt a concentration of 0.3% ?9-THC (dry weight basis) in young, vigorous leaves of relatively mature plants as a guide to discriminating two classes of plants.”

It is also significant that the researchers measured THC concentration from the plants’ leaves and not their flowers, which are widely known to have a higher cannabinoid concentration. (The USDA currently requires testing the flower portion of hemp.)

Many farmers and other industry stakeholders have strongly opposed the mandated 0.3% THC limit in public comments on the USDA’s interim final rule on hemp. U.S. Sen. Rand Paul (R-KY) is attempting to address the issue through the Hemp Economic Mobilization Plan Act of 2020, which he introduced in mid-December, to increase the limit to 1%. Many industry and advocacy groups support the legislation, including Vote Hemp, the American Farm Bureau Federation, and the Kentucky Hemp Association.

Vote Hemp also launched a petition it plans to share with Congress early in the new session to push legislators to change hemp’s legal THC limit to 1%. (As of early January 2021, the petition had 7,500 signatures, according to Vote Hemp President Eric Steenstra.) The USDA set a deadline of Nov. 1, 2021, to issue final regulations, and many in the industry hope their comments will not fall on deaf ears.

Hemp Grower sought out world-renowned cannabis and endocannabinoid scientist, Dr. Ethan Russo, M.D., to help shed light on this THC threshold issue.

Russo is a board-certified neurologist whose interest in medicinal plants and ethnobotany steered him to cannabis. “I have always been interested in medicinal plants and began active ethnobotanical research in 1990,” Russo tells Hemp Grower. “After a sabbatical in the Amazonian rainforest in Peru, I decided to dedicate the rest of my career to this pursuit, and this led to cannabis investigation in 1996.”

His cannabis background ranges from consulting for GW Pharmaceuticals (the company behind Epidiolex, the first and only FDA-approved CBD prescription medication) to serving as past president of the International Cannabinoid Research Society and currently as founder/CEO of CReDO Science, a company “devoted to research and development of disruptive technologies affecting the endocannabinoid system.” He has also written numerous articles and books.

Here, Russo talks with Hemp Grower about THC’s intoxicating effects, how individuals’ endocannabinoid systems respond differently to dosages, and why 1% is an “absolutely scientifically defensible limit” for THC content.


Noelle Skodzinski: What are your thoughts on THC limits in hemp?

Dr. Ethan Russo: THC concentration is somewhat irrelevant. Even at 0.3% THC, if you have got enough of a dose, it could be intoxicating. But, let’s say someone is smoking cannabis with 1% THC. They could smoke a little without getting high and would likely stop before getting high because they would get sick from the smoke. It is widely acknowledged that 1% material is ineffective via inhalation to produce intoxication.


Skodzinski: So then, what do you think of the 1% THC limit many seek for hemp?

Russo: I fully endorse the 1% THC limit. It has generally been the scientific finding that 1% or less is not intoxicating. A recreational user would not be happy with it, regardless of how much they consume. It is an eminently reasonable and absolutely scientifically defensible limit. It is not going to get people high.


Skodzinski: What about concentrates or edibles? Are those also not going to get people high if derived from hemp with 1% THC content?

Russo: Yes, it is true that even less potent material can be extracted and concentrated to produce a psychoactive product, but the process is inefficient and not cost-effective, so most people would not pursue it. This explains the move toward high-potency material for the recreational market.


Skodzinski: How much would someone have to consume (tolerance and endocannabinoid system activity all considered) to get high from 1% THC, if it is even possible?

Russo: Mathematics provides an answer. 2.5 milligrams of THC is a threshold dose for most people without tolerance, while 5 mg is a moderate dose, and 10 mg is too much at once in the absence of tolerance. 250 mg of 1% THC material that is ingested would provide 2.5 mg of THC (assuming complete absorption, which is not realistic); 500 mg of 1% THC material would yield 5 mg [of THC], and 1000 mg of 1% THC material would yield 10 mg of THC.


Skodzinski: You have researched cannabinoids and terpenes and their interactions extensively. How important are other cannabinoids and terpenes in THC’s psychoactive and/or intoxicating effects? Are certain cannabinoids and/or terpenes known to tame or elevate THC’s psychoactive effects? And specifically, what do you know about CBD’s influence on THC’s effect?

Russo: Delta-9-THC is [the] most psychoactive, while delta-8-THC is probably comparable, and cannabinol (CBN) about 25% the potency. None of the others are intoxicating, but CBD and CBG [cannabigerol] are certainly psychoactive by virtue of their modulating effects on mood. CBD blunts the peak high of THC, delays its onset, but prolongs its overall effect.

Limonene, alpha-pinene and linalool are the most prominent terpenes in modulating the intoxicating effects of THC, while myrcene will compound its sedative effects. The modulating effects of terpenoids on the cannabis experience can be profound if adequately present, particularly with inhalation.

People interested in these issues should read my articles in greater detail, available at ethanrusso.org.



Skodzinski: Do you feel further research is needed regarding THC limits and classifying hemp?

Russo: THC dosing should be based on science, period. It is not plutonium. It can have side effects, but not fatal overdoses.

It was established in the early 1990s by Miles Herkenham that there are very few cannabinoid receptors in the cardio-respiratory centers of the brain in the medulla, and this explains why no matter what the dose of THC, it cannot depress respiration in the manner that opiates do.


Skodzinski: Do you feel the UN’s recent vote to remove cannabis from a list of the world’s most dangerous drugs will significantly impact cannabis research?

Russo: The UN vote will only aid research if American and other governments de-schedule cannabis and rewrite restrictive laws.



Noelle Skodzinski is editorial director of Hemp Grower, Cannabis Conference and sister magazines Cannabis Business Times and Cannabis Dispensary.