Hexahydrocannabinol (HHC): a potentially potent challenge for Ireland’s early intervention in psychosis network

To the Editor

We read with interest the case series published by O’Mahony et al. (2024) concerning two cases of hexahydrocannabinol (HHC) induced psychotic illnesses.

We contend that a combination of three factors, the illegality of tetrahydrocannabinol (THC) in Ireland, the decline in the perceived harmfulness of cannabis in the general population (Chiu et al., 2022) and the increasing prevalence of vaping amongst young people (Pinho-Gomes et al., 2023), may conspire to generate a potent public health issue, particularly regarding incident cases of psychotic disorders. HHC became widely available in the United States in mid-2021 and was first recorded in Europe in May 2022 (European Monitoring Centre for Drugs and Drug Addiction 2023). Due to concerns about both its known and potential risks, many European countries have prohibited or restricted its use (Holt 2024), but not Ireland, where it not a Schedule-controlled drug. Apart from the risk to individuals who consume inhaled cannabinoids, the sale of edible cannabis preparations, such as gummies and sweets, has been linked with acute THC toxicity in young children (Mattimoe et al., 2021). The extension of HHC into this market extends the potential population at risk.

There may be a particular risk to indivdiuals who already habitually consume THC. It is not possible to say whether HHC is more potent or tends to be consumed in larger quantities than THC. Due to the nature of illicit substance use, it can be difficult to be precise about the relative quantities of each when individuals switch from one to another. While a greater quantity of HHC relative to THC may need to be ingested to consume an equivalent level of the 9β epimer, the isomer responsible for the cannabimimetic effect (Ujváry, 2024), it is possible that the ease of obtaining HHC, its relatively affordability, odourless character and the convenience of vaping it may facilitate increased consumption and counteract any decreased potency. The lack of knowledge and public discourse about its effects may also contribute to a perception that it is a safer alternative to THC.

Public health monitoring infrastructure in Ireland should continue to be vigilant in the coming months. There is often a lag between initial availability, widespread adoption, and the cumulative exposure required before cases of substance induced psychotic disorders will begin to appear at scale. Ireland has a network of Early Intervention in Psychosis services covering a portion of the population, who carry out comprehensive clinical assessments, and may be well placed to identify any emerging trend.

Cleary more research is required in this area, however we echo the authors’ recollection of the days of headshops and ‘legal highs’, which occurred in a space where legislation lagged accumulating clinical observation. Psychotic illnesses occur at a crucial developmental junction for young people, have lifelong implications, and cannabis use has a clear impact on onset, severity and relapse (Hasan et al., 2020). We join the call for increased research, education, and legislative scrutiny in this area.