A major new study from Germany has found that people who smoke - or have smoked in the past - are significantly more likely to experience depression compared with those who have never smoked.

The research, conducted by the Central Institute of Mental Health (CIMH) in Mannheim, analyzed data from the German National Cohort, the country’s largest population-based health study. The findings were published in the journal BMC Public Health.

According to the study, the risk of depression rises with the number of cigarettes smoked per day. Researchers also found encouraging evidence that quitting smoking can reduce this risk over time, with longer periods of abstinence linked to a lower likelihood of depression.

Smoking is already recognized by the World Health Organization as the leading cause of preventable premature deaths worldwide, responsible for more than eight million deaths each year. While scientists have long suspected a link between smoking and mental health problems, the biological and social mechanisms behind this connection remain unclear.

To explore this further, researchers examined how smoking intensity and the length of time since quitting influenced depression risk. The study involved nearly 174,000 participants aged between 19 and 72, with women making up half of the group. Participants were categorized as never-smokers (around 82,000), former smokers (58,000), and current smokers (34,000).

They were surveyed about lifetime and current symptoms of depression, along with detailed smoking histories, including age at smoking initiation and daily cigarette consumption.

The results showed that both current and former smokers reported higher rates of depression over their lifetime than those who had never smoked. The association was strongest among individuals aged 40 to 59, suggesting that the relationship between smoking and depression may vary with age and be influenced by social and time-related factors.

Importantly, the study found that people who quit smoking many years earlier had longer periods without depressive episodes compared to those who had stopped more recently, underscoring the mental health benefits of long-term smoking cessation.

Dr Fabian Streit of CIMH said the findings reinforce the importance of preventing smoking initiation and expanding support for individuals trying to quit, noting that stopping smoking may play a meaningful role in improving mental well-being.