"Smoking affects the brain," said Bogdan Petre, lead author of the study and a technical scientist at Northwestern University Feinberg School of Medicine.

"We found that it affects the way the brain responds to back pain and seems to make individuals less resilient to an episode of pain," Petre said.

This is the first evidence to link smoking and chronic pain with the part of the brain associated with addiction and reward, researchers said.

The results come from a longitudinal observational study of 160 adults with new cases of back pain.

At five different times throughout the course of a year they were given MRI brain scans and were asked to rate the intensity of their back pain and fill out a questionnaire which asked about smoking status and other health issues.

Thirty-five healthy control participants and 32 participants with chronic back pain were similarly monitored.

Scientists analysed MRI activity between two brain areas (nucleus accumbens and medial prefrontal cortex, Nac-mPFC), which are involved in addictive behaviour, and motivated learning.

This circuitry is critical in development of chronic pain, scientists found.

These two regions of the brain 'talk' to one another and scientists found that the strength of that connection helps determine who will become a chronic pain patient.

By showing how a part of the brain involved in motivated learning allows tobacco addiction to interface with pain chronification, the findings hint at a potentially more general link between addiction and pain.

"That circuit was very strong and active in the brain's of smokers," Petre said.

"But we saw a dramatic drop in this circuit's activity in smokers who - of their own will - quit smoking during the study, so when they stopped smoking, their vulnerably to chronic pain also decreased," Petre added.

The study was published in the journal Human Brain Mapping.

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