You may have heard that loneliness is as bad as smoking 15 cigarettes a day. Where does this idea come from? In 2010, researchers looked at 148 studies with about 300,000 people. People with strong social ties were about 50 percent more likely to survive during the studies.

But the "15 cigarettes" idea is only a comparison to make the result easy to understand. It is not an exact fact.

The phrase "loneliness is as harmful as smoking 15 cigarettes a day" is often repeated, but what is it based on? In 2010, psychologist Julianne Holt-Lunstad and her colleagues analyzed 148 studies covering about 300,000 people. They found that people with strong social relationships were about 50 percent more likely to survive during the study periods than those with weak relationships. The researchers said this effect was comparable to well-known risks such as smoking.

However, "15 cigarettes" is just a way of explaining the size of the effect. Researchers also separate social isolation, which means actually having few contacts, from loneliness, which is the feeling of not being connected enough. Both have been linked to higher risk of death.

A popular slogan holds that loneliness is as harmful as smoking fifteen cigarettes a day, and in 2023 it appeared in an advisory from the U.S. Surgeon General. Its scientific basis is a 2010 meta-analysis by Julianne Holt-Lunstad and colleagues, covering 148 studies and roughly 300,000 people. People with stronger social relationships had about a 50 percent greater likelihood of survival, an effect the authors called comparable to well-known risk factors such as smoking and drinking. The cigarette comparison conveys the size of that effect. It does not claim that loneliness works on the body like smoking.

Researchers also separate social isolation, an objective lack of contact, from loneliness, the subjective feeling of not being connected enough. A 2023 analysis of 90 cohort studies with about 2.2 million participants linked isolation to roughly 1.3 times the risk of death and loneliness to about 1.1 times, so the actual lack of contact showed the stronger association.

All of these studies are observational. Poor health can itself cut people off from others, and other confounders may play a part. Proposed pathways include heightened stress responses, worse sleep and exercise habits, and having nobody to push them toward medical care.