Plain language summary: The Department of Health asked the Health Information and Quality Authority (HIQA) to look at research to help update Ireland’s low-risk alcohol guidelines. Lowrisk alcohol guidelines give people information on how to lower their chances of getting hurt or sick from alcohol. This report looked at how drinking alcohol might impact someone’s mental health later in their life. We looked at the different ways that people drink alcohol, including how much people drink, how often people drink, patterns of drinking (for example, binge drinking – when someone drinks a lot of alcohol in a short time), and at what age people start drinking. Australia and Canada have done similar reports on this topic before. We updated the work completed by Canada. We looked for studies that explored if drinking alcohol could lead to depression, anxiety or suicidal thoughts, later on in life. Because only a few studies looked at suicidal thoughts later on in life, we also included studies that measured drinking alcohol and having suicidal thoughts, at the same time. 

We included 104 studies and found that the research linking the different ways that people drink alcohol, and their mental health, was not very clear. This is because studies measured how much alcohol people drink and mental health differently, which made the results hard to compare. The studies also had other issues that make it hard to draw conclusions. Many studies asked people to remember and report how much alcohol they drank in the past, which may not always be accurate or reliable. Some studies did not follow people for a long enough time to accurately check if there was an impact. Nearly all of the studies were done outside Ireland, making it hard to compare the results to Ireland.  We also had low or very low confidence in the results of the studies included because some of the studies had issues with how they were done, results didn’t always agree across studies, and some findings weren’t very precise. These issues make it difficult to clearly show how alcohol consumption patterns impact mental health. Keeping this in mind, the results for depression, anxiety and suicidal thoughts are presented below. 

Depression:

• In 17 studies, which had results we have low confidence in, there was not a clear link between how much alcohol adults (18 years and older) drink and developing depression later in life. Another 26 studies, which provided results we have low or very low confidence in, found mixed results on whether how often people drink, and binge drinking, are linked with developing depression later in life.
• In older adults (65 years and older), 15 studies, which provided results we have very low confidence in, found mixed results on whether how much people drink is linked with developing depression later in life. 
• In adolescents (under 18 years old), 21 studies, which provided results we have very low confidence in, did not show a clear link, or showed there may be a small connection between how much or how often people drink alcohol (including binge drinking), and developing depression later in life.

Anxiety:

• In adults, four studies, which provided results we have low or very low confidence in, found mixed results on whether how much people drink, and how often people drink (including binge drinking), are linked with developing anxiety later in life.
• In adolescents, eight studies which provided results we have low confidence in, did not show a clear link between how much people drink, how often people drink (including binge drinking), and what age people start drinking at, and developing anxiety later in life. 

Suicidal thoughts:

In adults and adolescents, the studies included provided results we have very low confidence in:
o In adults, two studies that followed people over time, showed that people who drink alcohol may be a small bit more likely to have suicidal thoughts later in life. Thirteen studies showed that drinking more alcohol may mean a person is more likely to have suicidal thoughts, when measured at the same time.
o In adolescents, one study that followed people over time, did not show a clear link between binge drinking and suicidal thoughts later in life. Eight studies showed that drinking more alcohol may mean a person is more likely to have suicidal thoughts, when measured at the same time.

Overall, the studies were done in many different ways, which makes it hard to compare them and know for sure how drinking alcohol affects mental health. In Ireland, binge drinking is very common, especially among young people. In Ireland alcohol is often linked with worse mental health and is often involved in self-harm and suicide. So, drinking alcohol and mental health are relevant public health concerns.

This review provides policy-makers with an overview of the international research on the relationship between drinking alcohol and the impact on mental health. This review together with other research and other evidence from the Irish setting will inform the update of Ireland’s low-risk alcohol drinking guidelines.