New Study on D-Limonene Starts Unraveling the Mystery of the Entourage Effect

A new study finds that combining d-limonene with THC reduces the THC-induced anxiety some people feel, offering tentative support for the entourage effect.

Written by

Lee Johnson

Lee Johnson is the senior editor at CBD Oracle, and has been covering science, vaping and cannabis for over 10 years. He has a MS in Theoretical Physics from Uppsala...

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Key Takeaways

  • Researchers gave 20 occasional cannabis users combinations of different doses of THC and a terpene called d-limonene.
  • The results showed that the participants felt less anxious when 15 mg of d-limonene was combined with 30 mg of THC, compared to just the THC alone.
  • The study offers some evidence for the “entourage effect,” but it is one piece of a big puzzle that will take much more research to solve.

Weed is more than just a vehicle for THC.

Legendary cannabis researcher Dr. Ethan Russo published one of the most influential papers about the “entourage effect” back in 2011.

He explained to CBD Oracle that “Debate has raged in the scientific community for decades between those that have claimed that effects of cannabis are solely attributable to THC, or more recently, perhaps CBD, versus those that have recognized cannabis as a true botanical medicine with many components that act in concert to produce the overall pharmacological benefits.”

The problem is that despite how influential this idea is, it has barely been tested; it’s more like stoner hearsay.

But Dr. Russo, alongside lead author Dr. Tory Spindle and colleagues, recently took a big step towards a solution to this mystery, in the form of a new study on d-limonene.

This is a citrusy terpene found in cannabis that is believed to have an anxiety-reducing entourage effect, and now there’s hard evidence to back this up.

Dr. Russo continued, “This study supports the botanical concept, commonly known as ‘the entourage effect,’ as first proposed by Professors Mechoulam and Ben-Shabat a generation ago.”

The Study: What They Did

The study had a pretty simple design.

First, they recruited healthy participants aged 18 to 55, who occasionally used cannabis but hadn’t used other illegal drugs in the past three months. These participants were then randomized to receive some combination of d-limonene and THC from a vaporizer, with some just having one substance and some combining them.

There were 9 dosage combinations initially (1 or 5 mg of d-limonene, 15 or 30 mg of THC, combinations of these and a placebo), and all participants received these in a random order to minimize issues with things like tolerance.

In the end, the researchers added a final condition (30 mg of THC and 15 mg d-limonene) when they had confirmed that lower doses of d-limonene were safe. Overall, they ended up with 20 participants for the 9 initial sessions, and 12 continued for the 10th session.

The researchers looked at reported anxiety levels and “drug effects” in general in each of these sessions, as well as taking physical measurements of heart rate, blood pressure and the quantities of THC and d-limonene in their blood.

Broadly, the plan was to compare their level of anxiety after taking different THC-to-d-limonene ratios.

The Results: D-Limonene Reduces THC-Induced Anxiety

Lead author Dr. Tory Spindle explained to us that, “The most important result was that we saw a mitigation of THC-induced anxiety by co-administering limonene, and this effect was dose-dependent.”

This main result is shown in Figure 1 of the paper, with ratings for participants’ levels of anxiety and paranoia notably decreasing with dosage when 30 mg of THC was paired with either 5 mg or 15 mg of d-limonene.

While there were clear signs that higher d-limonene doses made participants less likely to feel as though their heart was racing, the comparisons for anxiety, paranoia and “unpleasant drug effect” between 30 mg THC alone and THC in combination with 15 mg of d-limonene were the only statistically significant results.

Dr. Spindle continued, “This is one of the first demonstrations of the cannabis entourage effect in humans, which is very important because there is little to no empirical clinical data to support many claims made by the industry regarding the influence of minor cannabinoids/terpenes on cannabis effects.”

Does the Study Prove the Entourage Effect Is Real?

The short answer is that it is a strong piece of evidence for the entourage effect when it comes to d-limonene and THC, but it’s a little premature to assume that the entourage effect more broadly is scientifically proven.

We asked the researchers if the fact that d-limonene alone didn’t seem to have an effect on anxiety offers evidence for the entourage effect.

Dr. Russo commented to us, “Yes, it does. At inhaled low doses, the effect of limonene was too subtle to show statistical significance between groups. However, in combination with THC, the changes were clear. This is part of the definition of synergy of ingredients, wherein the whole (plant) is greater than the sum of its parts (single isolated compounds).”

However, Dr. Ryan Vandrey, Professor of Psychiatry and Behavioral Sciences at Johns Hopkins University, and supervising author on the paper, made an important point, “This experiment is specific to how co-administration of delta-9-THC and d-limonene affect anxiety. While this may be part of a larger ‘entourage effect,’ such an effect is not operationally defined and involves much more complex chemical interactions beyond what was evaluated in this study.”

As is often the case with science headlines, while you might want to jump right to “the entourage effect is real” the actual conclusion is more like “this offers evidence for the entourage effect in this limited context and more study is needed to see how applicable this is to real life and the broader theory.”

Is There Any Real-World Significance?

According to the research, d-limonene was most effective at reducing THC-induced anxiety at a 15 mg dosage, relative to 30 mg of THC.

However, this presents an issue, which Dr. Vandrey explained to us:

“We observed a significant decrease in delta-9-THC induced anxiety with a d-limonene dose that was 3 times the maximum amount we would expect to find naturally in cannabis flower based on chemical analysis of over a 100 different samples of botanical cannabis. There was a qualitative reduction in anxiety at the max d-limonenene dose we would expect in cannabis flower, but it was not statistically significant.”

So while there was some apparent effect at the highest natural d-limonene dose, the results of this study suggests that the most effective dose is beyond what would be found naturally.

However, Dr. Vandrey pointed out that other cannabis chemicals could have a similar effect, and that, “a more robust effect of smaller doses of d-limonene could happen in a different population of users.”

Those other cannabinoids and terpenes present in real-world cannabis also present a potential issue.

Dr. Russo commented to us that, “Cannabis contains upwards of 500 different chemical compounds. It becomes very difficult to demonstrate salient differences in the face of so many variables.”

Imagine being sick with a mystery illness and taking five different treatments – if you get better, how can you know which one helped? This is the core of the issue, and also why the study focused specifically on d-limonene rather than going right to the 500-molecule cacophony of cannabis.

Dr. Russo explained the approach, “The scientific approach here called for what I like to call a ‘deconstruction/reconstruction process,’ by which we can parse the effects of more simple combinations to demonstrate synergy.”

Simply put, this is one step in a long process of “building up” the entourage effect by checking the different connections individually.

In terms of the analogy, first you try antibiotics for the mystery illness, then you wait and see if it works before taking something else. Then you find out what helps and what doesn’t, and eventually you’ll see what exactly made your five-pill cocktail work.

Conclusion – What It All Means and the Next Steps

Overall, the study gives evidence for something cannabis consumers have been arguing for years – terpenes like d-limonene modulate the effects of THC.

However, this is the first step in an ongoing process and there is a lot we still don’t know.

Dr. Spindle explains, “We have several studies planned or ongoing to extend this line of research including studies on interactions between THC and other terpenes (pinene and myrcene) and a study to evaluate oral THC and oral limonene combinations (the preferred method among those who use cannabis for medical purposes).”

Dr. Russo also points out that while most consumers might not see cannabis that offers the most effective dose in the study, “these chemovars certainly do exist, and selective breeding for limonene can make this a reality now that we have demonstrated the effect and therapeutic benefit of reducing THC-induced anxiety by selecting for limonene content and that of other useful components.”

So while there is more to find out, even this result may inspire growers and geneticists to produce cannabis with enhanced protection against THC-induced anxiety.

But as always, we can’t be exactly sure where this road will lead, we just know we’re learning more with every step we take.