Although numerous studies have shown there is an association, it has been difficult to disentangle whether being lonely leads to substance abuse, or if substance abuse leads to loneliness.

By applying a novel research method to the question - Mendelian randomisation - which uses genetic and survey data from hundreds of thousands of people, the team found that loneliness appears to lead to an increased likelihood of smoking behaviour.

“This method has never been applied to this question before and so the results are novel, but also tentative. We found evidence to suggest that loneliness leads to increased smoking, with people more likely to start smoking, to smoke more cigarettes, and to be less likely to quit,” said co-lead author, Dr Robyn Wootton.

There was evidence that being lonelier increases the likelihood of starting smoking, the number of cigarettes smoked per day and decreases the likelihood of successfully quitting. This reflects the trends observed during the pandemic - YouGov’s Covid-19 tracker suggests 2.2 million people across the UK are smoking more than they were before lockdown. In the other direction, there was also evidence that starting smoking increased individuals’ loneliness.

The study was co-led by the Tobacco and Alcohol Research Group (TARG) in Bristol's School of Psychological Science and the department of Psychiatry of Amsterdam UMC hospital in the Netherlands, part of the Medical Research Council funded Integrative Epidemiology Unit.

“Our finding that smoking may also lead to more loneliness is tentative, but it is in line with other recent studies that identified smoking as a risk factor for poor mental health. A potential mechanism for this relationship is that nicotine from cigarette smoke interferes with neurotransmitters such as dopamine in the brain,” said senior author Dr Jorien Treur from Amsterdam UMC.

Deborah Arnott, chief executive of Action of Smoking & Health (ASH), commented that, “If lonely people are more likely to start smoking and find it harder to quit, they are more likely to suffer the harm caused by smoking. Smoking is the leading cause of preventable premature death, with thirty times as many people who die suffering serious smoking-related illnesses such as cancer, heart and respiratory disease. This research highlights the need for smokers suffering from loneliness to be given support to stop, to improve not just their health and wellbeing but also to help reduce their loneliness.”

The team also investigated the relationship between loneliness and alcohol use and abuse, and found no clear evidence for a causal relationship there.

“While the method that we used in this study has important advantages, it is also early days for this type of study. We therefore suggest further research is conducted when more is known about the genetics of alcohol dependence and loneliness,” said Dr Jorien Treur

According to the Office of National Statistics' (ONS) analysis of loneliness in Great Britain during the coronavirus (COVID-19) pandemic, during the first month of lockdown the equivalent of 7.4 million people said their well-being was affected through feeling lonely. "Lonely" people were more likely than others to be struggling to find things to help them cope and were also less likely to feel they had support networks to fall back on.

Knowing whether there is a causal effect of loneliness on smoking is important for informing stop smoking services and helping people to quit.

“Suddenly, the whole of the UK has become more socially isolated than ever before, and for many people this will likely increase their loneliness. We were really interested to find that loneliness decreases the likelihood of stopping smoking and we think this is a really important consideration for those trying to stop smoking during the pandemic,” said Dr Wootton.

“We are still yet to see the full effects of the coronavirus pandemic on alcohol and cigarette use in the UK.  Whilst our study does not look at the effects of loneliness and social isolation as the result of the pandemic, it can shed some light on the consequences of loneliness in general,” said Dr Wootton.