Long-Lasting Cognitive and Physical Impairment After Recreational Use of the Semisynthetic Cannabinoid Hexahydrocannabinonyl (HHC-C9): A Case Report

OG Article By Authors

• Nanna Reiter, Copenhagen University Hospital

• Dorte Fris Palmqvist, Copenhagen University Hospital

• Gro Borges Larsen, Kolding Hospital

• Mathilde Emilie Høi, Kolding Hospital

• Brian Schou Rasmussen, University of Copenhagen

• Ragnar Thomsen, University of Copenhagen

Published: 11 September 2025, Reports 2025, 8(3), 176; https://doi.org/10.3390/reports8030176

Abstract

Background: Semisynthetic cannabinoids (SSCs) like hexahydrocannabinonyl (HHC-C9) are increasingly used recreationally. They mimic THC but may cause stronger, prolonged effects. SSCs are sold as legal highs in forms like edibles, vapes, or herbal mixtures, often mislabeled to bypass regulations.Case: A man in his early forties consumed two cannabis cookies (labeled 40 mg “CC9”) and a gummy. He experienced vomiting, visual issues, and nine days of cognitive and physical impairment. HHC-C9 was detected in his blood.Conclusion: HHC-C9 can cause severe symptoms, including vomiting and drowsiness, requiring hospital care. Its varying potency and easy access pose risks.

Introduction

Semisynthetic cannabinoids (SSCs) mimic THC’s psychoactive effects but evade legal restrictions. Examples include HHC, THCP, and HHC-C9. Over 200 synthetic cannabinoids exist, with SSCs making up 40% of new substances reported in Europe in 2024 (EUDA, 2025). SSCs are sold online or in stores as edibles, vapes, or herbal mixtures, often labeled as “souvenirs” to avoid regulation. Their inconsistent potency increases intoxication risks.

Case Presentation

In May 2025, a healthy man in his early forties consumed two cannabis cookies (labeled 40 mg “CC9”) and part of a gummy, alongside alcohol, at a party. The next day, he experienced vomiting, cognitive issues, and reduced physical ability. Relatives noted disorientation, impaired alertness, and unsteady walking, contacting the Danish Poison Information Centre (DPIC).

DPIC recommended hospital admission. The patient showed delayed speech, mild disorientation, blurred vision, and deep somnolence. Tests (ECG, blood, urine) were normal except for mild leukocytosis. A drug screen was negative for common substances. Forensic analysis confirmed HHC-C9 in his blood. After 12 hours of observation, he improved but had persistent symptoms. Full recovery took nine days.

Toxicological Analysis

Blood and urine samples, taken 12 hours post-admission, were analyzed using liquid chromatography-mass spectrometry (LC-MS). HHC-C9 and two metabolites were detected in blood but not urine. Cookies of the same brand, purchased later, contained mostly HHC, with trace HHC-C9, suggesting batch inconsistencies.

Discussion

HHC-C9, a novel SSC, caused prolonged symptoms in this case, aligning with reports of extreme drowsiness and vision issues (Times of Malta, 2025). Unlike a case in Italy with agitation (Bini et al., 2025), this patient showed sedation. SSCs’ varying potency and appealing packaging (e.g., candies) increase risks, especially for young users.

Conclusion

HHC-C9 intoxication caused prolonged drowsiness, cognitive impairment, and visual disturbances. The growing use of SSCs, their inconsistent dosing, and easy availability highlight the need for more research and monitoring to understand their risks.

Funding

No external funding.

Ethics

Ethical approval was waived per Danish regulations. Patient consent was obtained.

Conflicts of Interest

None declared.