I've heard smoking causes back pain. Is this true?

There is a link between smoking and low back pain (LBP). This makes smoking a risk factor for LBP, but not the actual cause.

Most of the research shows indirect links. For example smoking can lead to chronic coughing which may be linked with back pain. Smokers tend to have other poor health habits. They may be inactive, overweight, and at risk for brittle bones (osteoporosis). Osteoporosis increases the risk of bone fracture.

A recent study compared muscle strength in smokers versus nonsmokers. There were two groups of each: those with back pain and those without back pain. In general, smokers have less lumbar muscle strength. This is true whether or not they have back pain.