Illicit Substance Use Linked to Increased Stroke Risk in Massive 100 Million Participant Study
A large-scale analysis involving data from over 100 million participants suggests a link between the use of several illicit substances and an increased risk of stroke. Genetic evidence from the study provides support for a potential causal role of substance use disorders in stroke incidence. The significant findings were published in the International Journal of Stroke.
Study Methodology
The comprehensive research consolidated data from 32 observational studies and incorporated a Mendelian randomization (MR) analysis. The meta-analysis encompassed over 100 million participants drawn from administrative, hospital, and population-based datasets, offering an unprecedented scale of investigation.
The MR analysis utilized genetic variants associated with specific substance use disorders to investigate potential causal relationships with various stroke outcomes. Seven proxy exposures were evaluated against different stroke phenotypes:
- Substance Use Disorder (SUD)
- Problematic Alcohol Use (PAU)
- Alcohol Use Disorder (AUD)
- Problematic Opioid Use (POU)
- Cannabis Use Disorder (CUD)
- Cocaine Dependence (CD)
- Nicotine Dependence (ND)
The study primarily focused on individuals of European ancestry. Dr. Megan Ritson, a research associate at the University of Cambridge, was identified as the lead author.
Observational Findings
The meta-analysis of observational data indicated clear associations between several substances and an elevated stroke risk:
- Cannabis use: Linked to an approximate 37% increase in overall stroke risk and a 39% increase for ischemic stroke.
- Cocaine use: Correlated with roughly double the risk of non-specific stroke, ischemic stroke, and hemorrhagic stroke. Other analyses showed a 96% higher stroke risk for cocaine users.
- Amphetamine use: Associated with significantly higher stroke risk, with pooled odds ratios around 2.37 for ischemic stroke and 2.83 for hemorrhagic stroke. Another analysis indicated a 122% higher stroke risk for amphetamine users.
- Opioid use: Did not show an overall association with increased stroke risk, with one analysis noting no evidence of heightened risk, and another indicating no overall association except with recent use. The latter finding was based on a small sample and requires cautious interpretation.
Specific risk patterns were observed for individuals under 55 years of age:
- Amphetamine users demonstrated a nearly threefold increase in stroke risk.
- Cocaine users exhibited a 97% increased risk.
- Cannabis users had a 14% additional risk.
- Opioid use was unexpectedly associated with lower stroke risk in this subgroup, which the authors attributed to selection bias and small sample sizes.
Genetic Evidence from Mendelian Randomization Analysis
The MR analysis provided genetic evidence linking certain substance use disorders to stroke, suggesting a potential causal role:
- Substance Use Disorder (SUD) variants: Predicted a 33% higher risk of any stroke and an approximately eight-fold higher risk of intracerebral hemorrhage.
- Cannabis Use Disorder (CUD) variants: Increased the risk of any stroke by 11% and large-artery stroke by 35%.
- Cocaine Dependence (CD) variants: Showed an 8% higher risk of cardioembolic stroke and a 38% increase in the risk of intracerebral hemorrhage.
- Problematic Alcohol Use (PAU) variants: Associated with approximately a 50% increase in cardioembolic stroke risk and about a doubling of large-artery stroke risk.
- Alcohol Use Disorder (AUD) variants: Linked to modestly increased risks of any stroke and cardioembolic stroke.
- Problematic Opioid Use (POU) variants: Associated with a modest increase in any stroke and ischemic stroke risk.
- Nicotine Dependence (ND) variants: Not associated with a change in stroke risk.
- MR analysis for amphetamines was not performed due to the absence of identified genetic variants.
Proposed Mechanisms
The study identified several potential mechanisms through which these substances may contribute to increased stroke risk:
- Substances such as cannabis, cocaine, and amphetamines are known to induce hypertension and cerebral vasospasm, potentially via sympathetic activation.
- Cocaine may also contribute to accelerated atherosclerosis.
- Cannabis is additionally noted to stimulate platelet aggregation, which could promote blood clot formation.
Study Strengths and Limitations
Strengths of the study include its comprehensive evaluation of multiple types of substance misuse and various stroke subtypes. The use of Mendelian randomization provides genetic evidence consistent with potential causal effects, reinforcing the observational findings.
Limitations include that most studies incorporated were from the USA, which may limit the generalizability of the findings. Many studies were based on hospital records, potentially introducing selection bias. Inconsistent reporting of exposure hindered adjustments for residual confounding factors, and the study population was limited to individuals of European origin.
Public Health Implications
The findings suggest that various forms of substance misuse are associated with an increased stroke risk. The study implies that these exposures should be considered during stroke risk assessment and highlights public health implications for prevention strategies targeting substance use disorders to potentially reduce stroke incidence.
Global recreational drug use has increased, with an estimated 300 million individuals reportedly consuming illicit drugs regularly. This includes 228 million using cannabis, 30 million using amphetamines, and 23 million using cocaine.
Juliet Bouverie, chief executive at the Stroke Association, stated that these substances can stress the cardiovascular system, leading to increased blood clotting, vessel narrowing, and circulatory damage.