THC levels in cannabis have increased over the last 50 years

New research shows that over the past 50 years, levels of THC have increased in cannabis.

The team behind the study analyzed data from more than 80,000 cannabis samples tested over the past 50 years and collected in the United States, the United Kingdom, the Netherlands, France, Denmark, Italy, and New Zealand. Their findings have been published in the magazine Addiction.

The researchers studied how THC concentrations had changed. In marijuana, they found that THC concentrations had increased by 14% from 1970 to 2017. Their explanation is based primarily on the fact that sinsemilia, female plants separated from the males to promote flower growth.

Researchers found that THC levels in the resin had increased by 24% between 1975 and 2017.

They also examined the concentrations of cannabidiol (CBD). Unlike THC, they did not observe any increase or decrease in CBD levels in cannabis over time.

The lead author of the study, the Dr. Tom Freeman, director of the Substance Use and Mental Health Group at the University of Bath, said: «As the potency of cannabis has increased, the number of people receiving treatment for cannabis use problems has also risen. There are now more Europeans seeking treatment for cannabis use than for heroin or cocaine.»

Sam Craft, a co-author of the study, explained: «Cannabis resin is often considered a safer form of cannabis, but our findings show that it is now more potent than «herb» cannabis.» Traditionally, cannabis resin contained much lower amounts of THC with equal amounts of CBD, but while CBD concentrations have remained stable, THC levels have increased significantly, meaning it is now much more harmful than it was many years ago.”

Researchers argue that the increase in the potency of cannabis highlights the need to implement broader harm-reduction strategies similar to those used for alcohol, such as standard units and public guidelines on limits for safer products.

Dr. Tom Freeman added: «As the potency of cannabis increases, users have limited information to help them monitor their consumption and make informed decisions about the relative benefits and risks. The introduction of a standard unit system for cannabis—similar to standard alcohol units—could help people limit their consumption and use it more safely.»

The research team has previously conducted various studies (here or here) and shown that frequent cannabis use with higher THC levels is associated with an increased risk of problems such as dependence, psychotic disorders, or cognitive impairment.