Genetic Analysis Suggests Cannabis Use May Increase Risk of Parkinson's Disease and ADHD in Women

What the researchers found

Using two-sample Mendelian randomization with GWAS data, the study found genetically predicted lifetime cannabis use was associated with increased risk of Parkinson's disease (OR=1.78, 95% CI: 1.03-3.08) and ADHD in women (OR=1.65, 95% CI: 1.05-2.59). No significant causal associations were found with Alzheimer's, ALS, epilepsy, migraine, schizophrenia, anorexia, or autism.

Why it matters

Traditional observational studies cannot easily separate whether cannabis use causes neuropsychiatric conditions or whether people predisposed to those conditions are more likely to use cannabis. Mendelian randomization uses genetic variation to approximate a natural experiment, providing stronger evidence about causal direction.

The numbers in context

Parkinson's disease OR=1.78 (95% CI: 1.03-3.08, p=0.038); ADHD in females OR=1.65 (95% CI: 1.05-2.59, p=0.029); no significant associations with AD, ALS, epilepsy, migraine, schizophrenia, AN, ASD, or MS

How the study worked

Two-sample Mendelian randomization using publicly available GWAS summary statistics. Genetic instruments for lifetime cannabis use tested against 10 neuropsychiatric disorders. Inverse variance weighted (IVW) method as primary analysis.

What this study cannot tell us

Mendelian randomization assumes genetic variants affect the outcome only through cannabis use (no pleiotropy). Moderate p-values that would not survive strict multiple comparison correction. Cannot assess dose-response, timing, or type of cannabis. European-ancestry GWAS limits generalizability.

How to read the evidence

Moderate: Mendelian randomization provides stronger causal inference than observational studies, but moderate p-values and standard MR assumptions apply.

When this study was published

2025 publication

The bigger picture

The Parkinson's finding is unexpected given that some cannabis compounds have been explored as neuroprotective agents. If confirmed, it would complicate the narrative around cannabis and neurodegeneration. The sex-specific ADHD finding adds to growing evidence that cannabis affects men and women differently.

Questions still open

• Through what mechanism could cannabis increase Parkinson's disease risk?
• Why would the ADHD association be sex-specific?

Read the original research

Association Between Cannabis Use and Neuropsychiatric Disorders: A Two-sample Mendelian Randomization Study.

Alpha psychiatry, 26(4), 46108

Citation

Guo, Wei; Dong, Lin; Lu, Qingxing; Xie, Mengtong; Yang, Yuqi; Zhang, Yanchi; Lu, Xiaoyu; Yu, Qiong. (2025). Association Between Cannabis Use and Neuropsychiatric Disorders: A Two-sample Mendelian Randomization Study.. Alpha psychiatry, 26(4), 46108. https://doi.org/10.31083/AP46108

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