Cannabis Science, CBD

Marijuana strength over the years: How much stronger is cannabis these days?


When someone speaks of the potency of weed, they are referring to the concentration of Δ9-tetrahydrocannabinol (THC). Have you ever heard people say that weed is stronger now than it was back then? Well, there may be some truth to that. Here is what the experts had to say.

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Has marijuana gotten stronger?

A study called Changes in Cannabis Potency over the Last Two Decades (1995-2014) – Analysis of Current Data in the United States examined 38,681 samples of cannabis preparations confiscated by the DEA. The cannabis preparations were predominately cannabis plant material (37,606).  There were also 814 samples of hashish. Hashish, according to Live Science, is the resinous parts of marijuana plants mixed with some plant particles. The researchers also looked at 261 samples of hash oil. Live Science describes hash oil as a concentrated oil form of marijuana. 

Examination of the samples showed a steady increase in marijuana potency over time. For example, in 1995 the potency was 4%, fast forward to 2014, the potency was 12%. According to How Stuff Works, THC typically ranges between 0.3 to 4 percent. However, the writer did note that some plants have THC concentrations of up to 25%. 

You can see a graph of how THC concentration has increased over the years. 

Graph showing Average Δ9-tetrahydrocannabinol (Δ9-THC) concentration of DEA specimens by year, 1995 – 2014 from Changes in Cannabis Potency over the Last Two Decades (1995-2014) – Analysis of Current Data in the United States

What was interesting was that the last four years of the study, there was an increase of sensimilla. If you don’t know, sensimilla is a highly concentrated form of marijuana.  According to the study “There is a shift in the production of illicit cannabis plant material from regular marijuana to sinsemilla”.

What about the other cannabinoids?

While THC has been steadily increasing, CBD doesn’t share the same fate. According to the study, “the CBD content has fallen on average from approximately 0.28% in 2001 to <0.15% in 2014”. This means that the ratio between THC and CBD have changed. The study states that there was a  “resulting in a change in the ratio of THC to CBD from 14 times in 1995 to approximately 80 times in 2014.”

You can observe the trend for CBD and other cannabinoids from the graph below.

Graph showing average concentration of Cannabichromen (CBC), Cannabidiol (CBD), Δ8-Tetrahydrocannabinol (Δ9-THC), Cannabinol (CBN), Cannabigerol (CBG), Tetrahydrocannabivarin (THCV), in DEA specimens by year, 1995 – 2014 (All cannabinoids except Δ9-THC). from Changes in Cannabis Potency over the Last Two Decades (1995-2014) – Analysis of Current Data in the United States

What do other studies say?

According to a public release by the American Chemical Society, a study was done in Colorado that examined 600 marijuana samples that were both recreational and medicinal. The researchers found that THC has been increasing over the years and CBD has been decreasing. Those were the same results found by the previous researchers.

According to Andy LaFrate, Ph.D, president and director of research of Charas Scientific and researcher of the Colorado study, “As far as potency goes, it’s been surprising how strong a lot of the marijuana is. We’ve seen potency values close to 30 percent THC, which is huge”.

Why is marijuana getting more potent?

Well, according to Dr Andy LaFrate, this could be due to intentional breeding. According to a YouTube video by the American Chemical Society, there could be cross-breeding done to increase the amount of THC. The video also noted that CBD levels have been very low. This proposition has been echoed by the researches of the study called Changes in Cannabis Potency over the Last Two Decades (1995-2014) – Analysis of Current Data in the United States. The study said, “the drug using community and the cannabis producers are breeding plants for the higher THC content”

According to Changes in Cannabis Potency over the Last Two Decades (1995-2014) – Analysis of Current Data in the United States, the increase in THC may be due to a shift from producing regular marijuana to sinsemilla. 

What are the consequences of an increase in marijuana potency?

There has been much debate on the effects of marijuana use on adolescents. According to Changes in Cannabis Potency over the Last Two Decades (1995-2014) – Analysis of Current Data in the United States says the increase in potency of THC spells bad news for adolescents using weed. 

The study says, “This increase in potency poses higher risk of cannabis use, particularly among adolescents.” The study mentioned that adolescents between the ages of 15-17 years old have been visiting the emergency department (ED) more. The study says, “In adolescents (aged 15-17 years old) it has been reported that ED visits involving marijuana use has risen in both males and females by 53.6% and 42.9%, respectively, from 2005 to 2010, which might be caused by the increase in cannabis potency over this time period.”

Nora Volkow, Director of the National Institute on Drug Abuse (NIDA) at the National Institutes of Health was quoted in Forbes agreeing that the increase in THC is leading to more emergency room visits. Volkow did not specify whether or not these were adolescents but just said this was happening all over the country. Her reasoning behind it is that a higher THC concentration will have a stronger effect on your brain. She mentioned that something like psychosis could result from that. What was interesting about what she said was that emergency room visits could befall even occasional users. 

According to Lewis Nelson, Professor and Vice Chair for Academic Affairs at NYU’s Department of Emergency Medicine and Director, of Medical Toxicology Fellowship at the New York City Poison Control Center who was quoted in Forbes, the rise in THC concentration isn’t really a concern. Lewis did say however that “the variability makes it more difficult to titrate the dose to the correct effect”.

Lewis expresses concern that cannabis users are more likely to consume too much which would lead to adverse clinical effects. He says that they have already started to see this at the Colorado emergency department. 

What should be done about the increasing potency of marijuana?

According to a journal article published in Missouri medicine called, “The Problem with the Current High Potency THC Marijuana from the Perspective of an Addiction Psychiatrist”, the potency should be controlled. The author posited that the potency of marijuana should ideally be less than 10%. The article said, “Ideally this would be to less than 10% as there is no good research on concentrations greater than this for any medical condition”. The author did make a note that there was no significant literature that discussed the negative effects of high potency THC.

What problems could a decrease in CBD concentration cause? 

Well, this could affect people who use marijuana for its medical benefits. CBD has many therapeutic effects and if the concentration is decreasing, and in some cases unable to be detected, then people are being robbed of that. 

Volkow, was quoted in Forbes saying while dosage instructions and labeling the marijuana would help there could still be a problem. She said, “but accurate information may not be known by the grower or seller, highlighting the inadequate regulatory state of recreational and medical marijuana.”


The research shows that it is true that THC concentration and thus marijuana potency has increased over the years. There will have to be some long-term studies to chronicle what exactly that means for users. As for right now, it seems to be causing more visits to the ER. It also increases the risk associated with using cannabis for adolescence. What seems to be important is regulation. When we say regulation we mean correct labeling of dosage, instructions, and proper testing to verify the information.


About Trevann

Trevann is Stoner Rotation’s Jamaica-based lead writer for the Science section of our cannabis blog. She graduated with honors receiving her Bachelor of Science degree in Molecular Biology from the University of West Indies, Mona. For the last three years, she has covered some of the biggest questions around cannabis and health underpinned with research from supporting studies, medical journals and scholarly articles. Got something on your mind? You can reach her at [email protected].