Cannabis is the third most used psychoactive substance worldwide. The legal status of cannabis is changing in many Western countries, while we have very limited knowledge of the public health impact of cannabis-related harms. There is a need for a summary of the evidence of harms and risks attributed to cannabis use, in order to inform the definition of cannabis risky use. We have conducted a systematic review of systematic reviews, aiming to define cannabis-related harms. We included systematic reviews published until July 2018 from six different databases and following the PRISMA guidelines. To assess study quality we applied the AMSTAR 2 tool. A total of 44 systematic reviews, including 1,053 different studies, were eligible for inclusion. Harm was categorized in three dimensions: mental health, somatic harm and physical injury (including mortality). Evidence shows a clear association between cannabis use and psychosis, affective disorders, anxiety, sleep disorders, cognitive failures, respiratory adverse events, cancer, cardiovascular outcomes, and gastrointestinal disorders. Moreover, cannabis use is a risk factor for motor vehicle collision, suicidal behavior and partner and child violence. Cannabis use is a risk factor for several medical conditions and negative social consequences. There is still little data on the dose-dependency of these effects; evidence that is essential in order to define, from a public health perspective, what can be considered risky use of cannabis. This definition should be based on quantitative and qualitative criteria that informs and permits the evaluation of current approaches to a regulated cannabis market.

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盲人和大象：对大麻使用相关健康危害的系统评价的系统评价

大麻是全球第三大最常用的精神活性物质。在许多西方国家，大麻的法律地位正在发生变化，而我们对大麻相关危害的公共卫生影响知之甚少。需要总结大麻使用造成的危害和风险的证据，以便为大麻风险使用的定义提供信息。我们对系统评价进行了系统评价，旨在定义与大麻相关的危害。我们纳入了截至 2018 年 7 月来自六个不同数据库并遵循 PRISMA 指南的系统评价。为了评估研究质量，我们应用了 AMSTAR 2 工具。共有 44 项系统评价，包括 1,053 项不同的研究，符合纳入条件。危害分为三个维度：心理健康、身体伤害和身体伤害（包括死亡）。证据表明，使用大麻与精神病、情感障碍、焦虑、睡眠障碍、认知障碍、呼吸系统不良事件、癌症、心血管结果和胃肠道疾病之间存在明显关联。此外，大麻的使用是机动车碰撞、自杀行为以及伴侣和儿童暴力的危险因素。大麻的使用是导致多种医疗状况和负面社会后果的危险因素。关于这些效应的剂量依赖性的数据仍然很少；从公共卫生角度定义什么可以被视为危险使用大麻所必需的证据。