View all news

Tobacco and alcohol may increase likelihood of using illegal drugs, new study shows

Press release issued: 30 September 2021

The use of legal drugs (tobacco and alcohol) may lead to the use of cannabis, a new study led by the University of Bristol and published in the journal Addiction has found. The study also found evidence that cannabis use may lead to smoking initiation, and opioid dependence could lead to increased alcohol consumption. Additionally, there might be shared risk factors that influence the use of multiple substances.

Dr Zoe Reed, Senior Research Associate in the Tobacco and Alcohol Research Group (TARG) in Bristol's School of Psychological Science and co-author of the paper, said: “Legal consumption of alcohol and tobacco may directly increase the level of illicit drug use. However, the relationships are complex. Consuming one drug does appear to increase the consumption of another, but it may also be the case that people have underlying risk factors which increase their chances of consuming both alcohol and tobacco and illicit drugs.”

The study found evidence for a possible gateway effect between:

  1. Tobacco use and subsequent alcohol and cannabis use
  2. Cannabis use and subsequent tobacco use
  3. Opioid dependence and subsequent alcohol consumption.

It is possible that there may be bidirectional relationships – for example between tobacco and cannabis use – where cause and effect is operating in both directions. However, given that tobacco and alcohol use usually begin before other drug use, it is also possible that there may be shared risk factors – possibly a common genetic predisposition to substance use – underlying these relationships. Further examination of these specific relationships is required to determine the exact mechanisms behind these possible gateway effects.

Hazel Cheeseman, Deputy Chief Executive of Action on Smoking and Health commenting on the studies implications said: “Tobacco and alcohol cause tremendous harm to society and these findings indicate that they may also increase the use of other drugs. Governments tend to take separate approaches to reducing the harm from legal and illegal drugs, but the long-promised Addictions Strategy provides an opportunity to look at the overlap between addictions and be more integrated.”

Many studies which have sought to understand the relationship between consuming different drugs rely on observed associations. The problem with relying on observed associations is that both types of drug use (legal and illegal) may be caused by a shared underlying risk factor, such as impulsiveness, and it is difficult to determine whether relationships are causal. To avoid this problem, the research team used a statistical approach called Mendelian randomisation.

Mendelian randomisation uses genetics to support stronger conclusions about possible causal relationships between an exposure (a potential cause) and outcome (a potential effect) that is less likely to be affected by a ‘confounder’ (a third variable that affects both the exposure and the outcome). In this study, researchers used known genetic variants that predispose people to use tobacco, alcohol, cannabis, cocaine or opioids and then looked at whether there might be causal relationships between both tobacco and alcohol and these illicit drugs. Using genetic variants as a proxy for an exposure reduces problems of confounding and allows stronger conclusions about whether an exposure truly causes an outcome – in this case whether use of one substance leads to the use of another.

The study was supported by Public Health England, Bristol's MRC Integrative Epidemiology Unit (IEU) and NIHR Bristol Biomedical Research Centre (BRC).


‘Using Mendelian randomisation to explore the gateway hypothesis: Possible causal effects of smoking initiation and alcohol consumption on substance use outcomes’ by Reed ZE, Wootton RE, and Munafò MR in Addiction

Further information

The paper is free to download for one month from the Wiley Online Library: or by contacting Jean O’Reilly, Editorial Manager, Addiction,

A short, animated introduction of the Mendelian randomisation technique is available on YouTube, see:

About the Tobacco and Alcohol Research Group (TARG)

TARG conduct research into the psychological and biological factors underlying health behaviours. They are part of the MRC Integrative Epidemiology Unit (MRC IEU) at the University of Bristol.

About MRC Integrative Epidemiology Unit

The MRC Integrative Epidemiology Unit (IEU) is a research unit based at the University of Bristol with funding from the Medical Research Council. It uses genetics, population data and experimental interventions to look for the underlying causes of chronic disease.

About the School of Psychological Science at the University of Bristol

The School of Psychological Science is a vibrant, friendly, and exciting place to work and study in. We offer accredited undergraduate and postgraduate taught courses, and have a lively and productive community of PhD students and research staff.

Our research spans developmental, social, cognitive and biological psychology, neuropsychology, and vision science. It has applications in areas ranging from human performance, memory, and communication to addiction and neurological disease.

Members of the School engage actively with the public and other potential audiences to share our passion for Psychology and the findings of our work.

About the NIHR

The mission of the National Institute for Health Research (NIHR) is to improve the health and wealth of the nation through research. We do this by:

  • Funding high quality, timely research that benefits the NHS, public health and social care;
  • Investing in world-class expertise, facilities and a skilled delivery workforce to translate discoveries into improved treatments and services;
  • Partnering with patients, service users, carers and communities, improving the relevance, quality and impact of our research;
  • Attracting, training and supporting the best researchers to tackle complex health and social care challenges;
  • Collaborating with other public funders, charities and industry to help shape a cohesive and globally competitive research system;
  • Funding applied global health research and training to meet the needs of the poorest people in low and middle income countries.

NIHR is funded by the Department of Health and Social Care. Its work in low and middle income countries is principally funded through UK Aid from the UK government.

About NIHR Bristol BRC

NIHR Bristol Biomedical Research Centre's (NIHR Bristol BRC) innovative biomedical research takes science from the laboratory bench or computer and develops it into new drugs, treatments or health advice. Its world-leading scientists work on many aspects of health, from the role played by individual genes and proteins to analysing large collections of data on hundreds of thousands of people. Bristol BRC is unique among the NIHR’s 20 BRCs across England, thanks to its expertise in ground-breaking population health research.

Edit this page