Irradiation and terpenes: what the measured data shows

September 2026, references expanded 21 September 2026

Cannabis is a plant product and is not inherently sterile. Spores and microbes are present on every flower. For an immunocompromised or chronically ill patient, inhaling harmful spores can have serious consequences. Aspergillus in particular is well documented as a risk: moulds are routinely cultured from cannabis flower, cannabis users in a large US insurance database were 3.5 times more likely to be diagnosed with a fungal infection than non-users, and fatal cases of invasive aspergillosis have been reported in immunosuppressed patients who inhaled cannabis.^1,2,3

A producer meets microbiological limits in one of two ways: through a controlled cultivation and post-harvest process that keeps contamination below threshold, or by irradiating. Gamma irradiation is effective, and it is the method used by several national medical cannabis programmes to meet pharmacopoeial microbial limits. Irradiation of food has been studied since the 1950s and authorised since 1963; a joint WHO, FAO and IAEA expert committee concluded in 1980 that food irradiated up to 10 kGy presents no toxicological hazard, and the EU harmonised food irradiation in Directive 1999/2/EC.^4,5 The question is not whether it works. It is what else it changes.

What the published data shows

The finding is consistent in direction: irradiation leaves cannabinoid content intact and reduces terpene content, with the more volatile monoterpenes affected most. The size of the terpene loss varies with dose, method and cultivar.

The 2016 study by Arno Hazekamp, which irradiated four Dutch pharmacy varieties at a minimum of 10 kGy, found no change in THC or CBD, in water content or in the microscopic structure of the flower, and no formation of degradation products. Terpene losses were generally between 10 and 20%, and as much as 38% for some components, mainly monoterpenes such as myrcene, ocimene and terpinolene; the profile was qualitatively unchanged, with no new compounds.⁶ A 2023 study on two tonnes of biomass irradiated at 15 to 21 kGy found no significant effect on any cannabinoid, only small and statistically non-significant changes in terpenes, and a reduction in yeast and mould counts to below 100 CFU/g.⁷

Other methods show the same pattern. Electron-beam treatment of hemp flowers in a 2025 study left cannabinoids unaffected and reduced total terpenes by 8% immediately after treatment (monoterpenes by 11%), but after twelve weeks of storage the untreated and treated flowers had lost 22% and 24% respectively, which means storage conditions matter at least as much as the treatment itself.⁸ X-ray irradiation at 2.5 kGy inactivated four Aspergillus species without significantly altering cannabinoid or terpene profiles,⁹ and gamma and e-beam treatments reduce fungal counts by five to six orders of magnitude.¹⁰ One e-beam study did find cultivar-dependent shifts in the cannabinoid content of extracts, a reminder that the effect is not identical for every variety.¹¹ Research on the effect of irradiation on minor cannabinoids remains sparse.

Patients who have used irradiated cannabis frequently report reduced smell and taste and a drier flower. Those observations are consistent with terpene loss, though the measured effect on water content is small or absent.^6,7

Why terpenes are of clinical interest

Terpenes are the aromatic compounds that distinguish one cultivar from another, and they are the reason two flowers at the same THC percentage are described differently by patients. Cultivar selection in medical cannabis is, in large part, terpene selection.

What the evidence does not show. To our knowledge no clinical trial has compared patient outcomes with irradiated against non-irradiated cannabis; the available data are analytical, on chemistry and microbiology, not clinical. The proposition that a fuller terpene profile improves therapeutic effect, the entourage effect, is a reasonable hypothesis with some mechanistic support, but it has not been established in controlled clinical trials, and the laboratory evidence is mixed: one study found common terpenes did not modulate THC’s activity at cannabinoid receptors at all, while another found that several terpenes produced cannabinoid-like effects in mice and enhanced a synthetic cannabinoid’s activity.^12,13,14 We are not going to tell you otherwise.

Motagon’s position

Every Mota® flower released to date has been non-irradiated. That is a sourcing and quality decision: we look for producers whose cultivation and post-harvest handling meet microbiological limits without gamma treatment, because doing so preserves the profile the cultivar was selected for.

It is not a claim that irradiated product is inferior medicine. Some of what we distribute is irradiated, and it is good product. Irradiation status is stated openly for every item we supply, so that the prescriber, not the distributor, decides what weight to give it.

What this means in practice

• If a patient stabilised on a cultivar reports a change in effect after a batch change, irradiation status is one variable worth checking alongside cannabinoid content, and so is the age and storage of the batch.
• Where response appears sensitive to cultivar rather than to THC content, terpene retention is a plausible factor and non-irradiated material is worth trialling.
• Irradiation status should never be the primary basis for product selection. Cannabinoid ratio, route of administration and patient tolerance come first.
• For immunocompromised patients, microbiological safety outranks terpene preservation.

References

1. Ruchlemer R, Amit-Kohn M, Raveh D, Hanuš L. Inhaled medicinal cannabis and the immunocompromised patient. Supportive Care in Cancer 2015;23(3):819-822. doi:10.1007/s00520-014-2429-3
2. Benedict K, Thompson GR, Jackson BR. Cannabis use and fungal infections in a commercially insured population, United States, 2016. Emerging Infectious Diseases 2020;26(6):1308-1310. doi:10.3201/eid2606.191570
3. Salam AP, Pozniak AL. Disseminated aspergillosis in an HIV-positive cannabis user taking steroid treatment. Lancet Infectious Diseases 2017;17(8):882. doi:10.1016/S1473-3099(17)30438-3
4. Joint FAO/IAEA/WHO Expert Committee. Wholesomeness of irradiated food. WHO Technical Report Series 659. Geneva: World Health Organization; 1981. See also US Food and Drug Administration, Overview of irradiation of food and packaging.
5. Directive 1999/2/EC of the European Parliament and of the Council of 22 February 1999 on the approximation of the laws of the Member States concerning foods and food ingredients treated with ionising radiation. eur-lex.europa.eu
6. Hazekamp A. Evaluating the effects of gamma-irradiation for decontamination of medicinal cannabis. Frontiers in Pharmacology 2016;7:108. doi:10.3389/fphar.2016.00108
7. Majumdar CG, ElSohly MA, Ibrahim EA, et al. Effect of gamma irradiation on cannabinoid, terpene, and moisture content of cannabis biomass. Molecules 2023;28(23):7710. doi:10.3390/molecules28237710
8. Goffman FD, Carrera DÁ, Latino DARS, Cronje C, Katsir L. Impact of electron beam treatment and storage duration on microbial stability and phytochemical integrity in hemp flowers. Molecules 2025;30(17):3601. doi:10.3390/molecules30173601
9. Frink S, Marjanovic O, Tran P, et al. Use of X-ray irradiation for inactivation of Aspergillus in cannabis flower. PLoS One 2022;17(11):e0277649. doi:10.1371/journal.pone.0277649
10. Jerushalmi S, Maymon M, Dombrovsky A, Freeman S. Effects of cold plasma, gamma and e-beam irradiations on reduction of fungal colony forming unit levels in medical cannabis inflorescences. Journal of Cannabis Research 2020;2:12. doi:10.1186/s42238-020-00020-6
11. Kovalchuk O, Li D, Rodriguez-Juarez R, Golubov A, Hudson D, Kovalchuk I. The effect of cannabis dry flower irradiation on the level of cannabinoids, terpenes and anti-cancer properties of the extracts. Biocatalysis and Agricultural Biotechnology 2020;29:101736. doi:10.1016/j.bcab.2020.101736
12. Russo EB. Taming THC: potential cannabis synergy and phytocannabinoid-terpenoid entourage effects. British Journal of Pharmacology 2011;163(7):1344-1364. doi:10.1111/j.1476-5381.2011.01238.x
13. Santiago M, Sachdev S, Arnold JC, McGregor IS, Connor M. Absence of entourage: terpenoids commonly found in Cannabis sativa do not modulate the functional activity of Δ9-THC at human CB1 and CB2 receptors. Cannabis and Cannabinoid Research 2019;4(3):165-176. doi:10.1089/can.2019.0016
14. LaVigne JE, Hecksel R, Keresztes A, Streicher JM. Cannabis sativa terpenes are cannabimimetic and selectively enhance cannabinoid activity. Scientific Reports 2021;11:8232. doi:10.1038/s41598-021-87740-8

Adapted in part from material originally written for Motagon by Lukáš Hurt. Reviewed by [name, role], Motagon s.r.o. Last reviewed 21 September 2026.