Age of first use matters because earlier onset is linked to cannabis use disorder and to negative consequences of use, but not consistently to other substance problems. Hamaoui et al. (2025), a systematic review of 16 prospective studies published between 2000 and 2024, found that earlier onset was associated with cannabis use disorder, and that association stayed independent of how often the teenager used. The same review found mixed findings for other substance problems, such as alcohol. Frequency still accounted for a significant part of the risk for negative consequences. Only 1 of the 16 prospective studies had low risk of bias.
Has cannabis use among young Iranians really risen?
Rostam-Abadi et al. (2021) pooled 90 studies and found strong evidence of a rise among youth. In the combined youth groups during 2016 to 2020, last-12-month cannabis use reached 4.9% of males (95% CI 3.4 to 6.7) and 0.3% of females (95% CI 0.0 to 1.3), and the linear trend from 2000 to 2020 rose significantly among males in that group.
The Iranian figures describe pooled youth groups; a photo of the plant says nothing about risk without age of onset and frequency — Photo: Jonathan Cooper / Pexels
The national general-population figures are lower. Last-12-month cannabis use was 1.3% of males (95% CI 0.1 to 3.6) and 0.2% of females (95% CI 0.1 to 0.3). Cannabis use disorder in national general-population studies rose from 0% in 2001 to 0.5% in 2011. Confidence intervals are wide, especially for females, and a pooled youth category is not a single nationally representative sample.
The authors describe Iranian use as low compared with many countries while calling the youth increase strong. The 4.9% figure describes a direction of change in a pooled youth category, not a precise national prevalence.
Why does the age of first use matter?
Hamaoui et al. (2025) reviewed 16 prospective studies from 2000 to 2024 and found that earlier onset was associated with cannabis use disorder and with negative consequences of use. The link to cannabis use disorder held independently of how frequently the teenager used. Findings for other substance problems, including alcohol, were mixed.
Our editorial position is clear: the key question is not whether a teenager has used cannabis; it is at what age, how often and at what potency the use began. Frequency of use accounted for a significant part of the risk for negative consequences. Of the 16 prospective studies, only 1 had low risk of bias, 7 had some concerns and 8 had high or very high risk of bias.
The practical implication drawn by the authors is that prevention should aim to delay onset and reduce frequency, because age of onset is the variable the prospective literature connects to a named diagnosis.
What difference does THC potency make for teenagers?
Hines et al. (2024) surveyed 6672 adolescents aged 13 to 14. High-potency cannabis, above 10% THC, was used by 2.6% (171 adolescents) and low-potency cannabis by 0.6% (38 adolescents). Compared with adolescents who had not used cannabis, high-potency users were more likely to report probable depression (odds ratio 1.59; 95% CI 1.06 to 2.39).
Associations with anxiety (odds ratio 1.45; 95% CI 0.96 to 2.20) and with auditory hallucinations (odds ratio 1.56; 95% CI 0.98 to 2.47) were also reported, but both intervals include 1, so neither reaches statistical clarity. Symptoms were self-reported, and the design cannot show that cannabis caused them.
Hambly Lapointe et al. (2026) mapped 71 included studies from 11225 identified, covering ages 14 to 25. Compared with low-potency cannabis, high-potency use above 15% THC was associated with a greater number and magnitude of adverse mental-health outcomes, including cannabis dependence, psychosis and cognitive deficits. [Needs further verification: the sources of this article contain no data on THC potency in cannabis used in Iran; all potency evidence here is international.] A scoping review does not pool effect sizes, so it describes direction rather than a measured magnitude.
Where should parents and schools start?
The starting point is the conclusion drawn by Hamaoui et al. (2025): prevention should aim to delay onset and reduce frequency of use. Schools can support that aim through credible information about potency and frequency rather than warnings framed around a single episode of use. Parents can do the same at home.
In the clinical experience of specialists who work with families of adolescents, a recurring pattern is that teenagers talk more about use in a calm, non-threatening conversation than in a confrontation after a discovery. A calm opening allows a parent to establish age at first use, frequency and potency, which are the three variables the prospective literature treats as relevant to later outcome.
As practical guidance, professional help is warranted when use is frequent, when school performance declines alongside withdrawal from family and peers, or when mood, sleep and thinking change in ways that persist after use stops. Escalating use despite clear consequences is another signal. Assessment is the appropriate step, and it does not require that a teenager agree with the concern.
Does a single use in adolescence lead to dependence?
The 2025 systematic review links earlier age of first use with cannabis use disorder, but it does not show that a single episode of use causes dependence. It reports a frequency relationship: frequency of use accounted for a significant part of the risk for negative consequences.
What does high THC potency mean?
The 2024 adolescent survey defined high potency as above 10% THC, while the 2026 scoping review used a threshold above 15% THC. The two bodies of evidence therefore define high potency differently, which limits how directly their findings can be compared.
What should I do first if my teenager has used cannabis?
Open with a calm conversation rather than a confrontation, which adolescents answer more openly. The systematic review on age of first use recommends delaying onset and reducing frequency; professional help is warranted when use escalates or when mood, sleep or thinking change.
Is cannabis use in Iran higher than in most countries?
Rostam-Abadi et al. (2021) reports that last-12-month cannabis use in the Iranian general population, 1.3% of males (95% CI 0.1 to 3.6) and 0.2% of females (95% CI 0.1 to 0.3), is low compared with many countries, while the youth trend rises.
Conclusion: What can be said with confidence?
The evidence supports a modest, useful claim: earlier onset is linked to cannabis use disorder, and in separate studies higher potency is associated with worse mental-health outcomes. Hamaoui et al. (2025) notes that only 1 of 16 prospective studies had low risk of bias, so the link is credible but not precise. Delaying onset and reducing frequency remain reasonable goals for families and schools.
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Rostam-Abadi Y, Gholami J, Amin-Esmaeili M, et al. Evidence for an increase in cannabis use in Iran – A systematic review and trend analysis. PLoS One. 2021. PMID: 34460847 · doi:10.1371/journal.pone.0256563
Hamaoui J, Pocuca N, Ditoma M, et al. Age of onset of cannabis use and substance use problems: A systematic review of prospective studies. Addict Behav. 2025. PMID: 39799660 · doi:10.1016/j.addbeh.2025.108259
Hambly Lapointe C, Nosko L, Crocker CE, et al. Influence of cannabis potency on mental health outcomes among adolescents and young adults: scoping review. BJPsych Open. 2026. PMID: 42410915 · doi:10.1192/bjo.2026.12023
Image Credits: featured image — Aleks BM / Pexels; first inline image — Jonathan Cooper / Pexels; the consulting-room image is AI-generated (Gemini 3.1 Flash via OpenRouter) and depicts no real person.
