Addiction and mental illness often go hand-in-hand. The association even exists at the less severe ends of the substance abuse and mental health spectrums. For example, a recent study found that individuals who have a history of anxiety disorders are more likely to struggle with nicotine addictions.

Researchers from the University of Wisconsin offered free substance abuse treatment to more than 1,500 smokers. The quit rate was generally high. However, individuals who had a history of anxiety disorder were significantly less likely to quit successfully. The findings support previous research, which found that as much as 20 percent of the 50 million U.S. smokers have some sort of anxiety disorder.

The conditions observed by researchers included a history of panic attacks, social anxiety and general anxiety. Participants who had experienced one or more of these conditions at some point in their lives were less able to quit, even with the help of nicotine gums or patches. They reported higher levels of dependency and experienced more severe withdrawal symptoms.

"Further research is needed to identify better counseling and medication treatments to help patients with anxiety disorders to quit smoking," said Megan Piper, who led the investigation.