Add another downside of lighting up to the list. The risks of smoking are well-documented, from the fact that a single cigarette shortens your life to weed not actually helping anxiety or depression . And while each substance has different neurological effects, new research has found that both marijuana and tobacco physically affect the brain, resulting in shrinkage. It’s long been known that aging is linked to a gradual reduction in brain volume , which is a key sign of degenerative conditions