Ireland's HHC Problem: What Happened When a Synthetic Cannabinoid Went Mainstream | Research

The takeaway

Over a third of Irish drug users had tried HHC (hexahydrocannabinol) before it was banned in 2025, with 90% reporting negative effects including anxiety, dissociation, and psychotic symptoms.

Drug policy researchers, harm reduction advocates, regulators dealing with novel psychoactive substances, and anyone considering synthetic cannabinoid products.

What the researchers found

Hexahydrocannabinol (HHC)—a semi-synthetic cannabinoid marketed as a legal alternative to cannabis—was openly sold in Irish shops until it was banned in July 2025. This study, using data from the 2024 European Web Survey on Drugs, captured the scope of the problem.

Among 2,314 Irish adults who had used drugs in the previous year, 36.2% reported lifetime HHC use, 33.5% used it in the past year, and 17.8% used it in the past month. These are remarkably high prevalence figures for a substance that most people had never heard of just a few years earlier.

The primary driver was accessibility: people started using HHC because it was easily available in high street shops (62.4% obtained it from a retail store). It wasn't sought out for unique effects—it was simply there, legal, and marketed as cannabis-like.

The harm data is concerning. Among HHC users, 89.9% reported at least one negative consequence. The most common: anxiety or panic reactions (14.7%), feeling faint or dizzy (13.4%), and dissociation (11.9%). Reports of psychotic illness precipitated by HHC use had already alarmed public health officials before the ban.

This case study illustrates the whack-a-mole nature of drug regulation: when one substance is controlled, close analogues fill the legal gap, often with less safety data and potentially more harm.

Why it matters

HHC's rapid rise and high harm rate demonstrates what happens when synthetic cannabinoids exploit regulatory gaps. Unlike well-studied THC, HHC had minimal safety data when it hit the market. The 90% harm rate among users is far higher than reported harm rates for natural cannabis, suggesting that "legal alternative" does not mean "safer alternative."

The numbers in context

N = 2,314. 36.2% lifetime HHC use. 33.5% past-year use. 17.8% past-month use. 62.4% obtained from high street shops. 89.9% reported negative consequences. 14.7% anxiety/panic. 13.4% faintness/dizziness. 11.9% dissociation.

How the study worked

Cross-sectional data from the 2024 European Web Survey on Drugs. 2,314 Irish adults aged 18+ who had used drugs in the previous year. Survey assessed HHC frequency, reasons for starting, sources, administration methods, and harms.

Who was studied

N=2,314 adults aged 18+, Ireland, drug users from an online survey

What this study cannot tell us

Web survey with self-selected participants who use drugs—not representative of the general population. Self-reported harms (no clinical verification). The 90% harm rate may reflect the survey's drug-using population rather than all HHC users. Irish-specific data may not generalize to countries with different regulatory environments. Recall bias for negative experiences.

How to read the evidence

Large web-based survey from the European drugs monitoring framework—good for capturing real-world use patterns but limited by self-selection and self-report.

When this study was published

Published in 2025 with 2024 survey data, capturing HHC use before Ireland's July 2025 ban.

The bigger picture

This is a cautionary tale relevant to the broader legalization discussion. RTHC-00181 found mostly neutral mental health effects from cannabis legalization, but synthetic cannabinoid analogues like HHC may carry very different risk profiles. The irony: prohibition of cannabis may have driven demand for HHC, which then produced worse outcomes than the substance it replaced. This pattern has been documented repeatedly with synthetic cannabinoids (Spice, K2) in other countries.

Replication

Not stated in abstract.

Funding

Not reported in abstract.

Conflicts of interest

Not reported in abstract.

Questions still open

Did banning HHC in Ireland reduce harms, or did users switch to another unregulated analogue? Would legal, regulated cannabis markets reduce demand for synthetic alternatives? How does HHC's safety profile compare to THC when both are used at equivalent doses?

Read the original research

Hexahydrocannabinol (HHC) use and harms in Ireland: New findings from the 2024 European Web Survey on Drugs.

The International journal on drug policy, 145, 105011

The International Journal on Drug Policy is a respected journal focusing on drug policy research.

View on PubMed ↗Publisher’s paper ↗

Citation

Mongan, Deirdre; Killeen, Nicki; Millar, Seán R; Matias, João; Keenan, Eamon; Galvin, Brian. (2025). Hexahydrocannabinol (HHC) use and harms in Ireland: New findings from the 2024 European Web Survey on Drugs.. The International journal on drug policy, 145, 105011. https://doi.org/10.1016/j.drugpo.2025.105011