THC in Rome's Hashish Nearly Doubled in 7 Years — From 13.7% to 27.1% | Research

The takeaway

Analysis of over 1,000 police-seized hashish samples in southern Rome showed average THC content nearly doubled from 13.7% in 2015 to 27.1% in 2022, with some samples exceeding 40%.

Read this if you use cannabis in Europe and assume it's the same strength it used to be — it almost certainly isn't.

What the researchers found

The potency escalation in cannabis isn't just a North American phenomenon. This study analyzed over 1,000 hashish samples seized by law enforcement in the southern Rome area between 2015 and 2022, documenting a dramatic increase in THC concentration.

Average THC content rose from 13.7% in 2015 to 27.1% in 2022 — nearly doubling in seven years. Some samples with unusual shapes and colors tested above 24%, and a few exceeded 40% THC. This is illicit market cannabis (Italy has not legalized recreational use), meaning these concentrations are being achieved without the quality controls or labeling of regulated markets.

The age group most associated with seizures was males aged 15-36, highlighting that the highest-potency products are reaching the demographic most vulnerable to cannabis-related mental health effects. The authors connect this potency trend to the growing body of research linking high-potency cannabis to psychosis, depression, anxiety, and cannabis use disorders in young adults.

The data also revealed that potency wasn't just increasing on average — the upper end of the distribution was pushing into territory that would have been almost unheard of a decade ago. These super-potent samples represent a qualitatively different product than what most research has studied.

Why it matters

Potency matters because dose determines risk. A joint of 27% THC hashish delivers nearly twice the THC of the same amount of 14% hashish. If health risks — psychosis, dependence, cognitive effects — are dose-dependent (and evidence increasingly says they are), then the same amount of cannabis use in 2022 carries roughly twice the risk it did in 2015. This European data point is critical because it shows potency escalation is a global trend, not a quirk of the U.S. or Canadian legal markets.

The numbers in context

Over 1,000 hashish samples analyzed. Average THC: 13.7% (2015) → 27.1% (2022). Some samples exceeded 40% THC. Most seizures involved males aged 15-36. Study period: 2015-2022 in the southern Rome area. This is illicit market cannabis — Italy has not legalized recreational use.

How the study worked

Observational analysis of over 1,000 hashish (cannabis resin) samples seized by law enforcement in the southern Rome area from 2015 to 2022. THC concentration was measured for each sample. Data analyzed for trends over time and by demographic characteristics of individuals involved in seizures.

Who was studied

N=1,000 hashish samples seized from 2015 to 2022 in southern Rome, Italy.

What this study cannot tell us

Law enforcement seizures are not a random sample of all cannabis in circulation — they may overrepresent larger quantities or specific trafficking routes. The study only analyzed hashish (resin), not herbal cannabis (marijuana), which may have different potency trends. No data on what consumers actually smoke per session (higher potency could lead to titration — using less per session). Southern Rome may not represent all Italian or European markets. No direct health outcome data — the link between rising potency and health effects is inferred from other research.

How to read the evidence

Observational analysis of forensic laboratory data from police seizures. The potency trend is clearly documented, but seizure samples may not perfectly represent what consumers are using. No direct health outcomes were measured.

When this study was published

Published in 2023 with data through 2022. Potency trends in the illicit market likely continue upward.

The bigger picture

This is the European companion to the potency trilogy already in the database. RTHC-00071 tracked rising THC in U.S. products, RTHC-00049 documented European potency trends, and RTHC-00086 showed that dispensary THC:CBD ratios overshoot therapeutic targets. This Rome data adds a time-series from the illicit market showing the same pattern. The potency escalation is happening everywhere — legal, illegal, North America, Europe — and it's accelerating.

Replication

Not stated in abstract.

Funding

Not reported in abstract.

Conflicts of interest

Not reported in abstract.

Questions still open

Do users of higher-potency hashish titrate their dose downward, partially offsetting the potency increase? How does the potency of illicit European hashish compare to regulated products in legal markets? Is the acceleration in potency (from ~14% to ~27% in just 7 years) approaching a ceiling, or will it continue? Should European drug policy respond to potency trends even in the absence of legalization?

Read the original research

Trends in Illicit Cannabis Potency based on the Analysis of Law Enforcement Seizures in the Southern Area of Rome.

Toxics, 11(8)

Toxics is a peer-reviewed journal that publishes research on toxic substances and their effects.

View on PubMed ↗Publisher’s paper ↗

Citation

Vernich, Francesca; Stefani, Lucrezia; Fiorelli, Denise; Mineo, Federico; Pallocci, Margherita; Treglia, Michele; Marsella, Luigi Tonino; Tittarelli, Roberta. (2023). Trends in Illicit Cannabis Potency based on the Analysis of Law Enforcement Seizures in the Southern Area of Rome.. Toxics, 11(8). https://doi.org/10.3390/toxics11080648