The changes in marijuana laws have been huge. Twenty-five states have now legalized it in some form and among those, four made it okay to smoke recreationally.

So, what are the long term effects of smoking marijuana? It's actually not well known, but a new study of New Zealanders who chronically used the plant for two decades found, surprisingly, that it's only associated with gum disease.

This study followed about 1,000 New Zealanders born in 1972 through age 38. They assessed the frequency of marijuana use and dependence at ages 18, 21, 26, 32 and 38 years old. Tests assessed such areas as gum or periodontal health, lung function, systemic inflammation, waist circumference, HDL cholesterol levels, and blood pressure. Marijuana use was compared to cigarette smoking as well as non-smoking controls. Of all the measures, the only negative effect observed, in about half the marijuana users with fifteen years of use, was increased gum disease.

Cigarette smokers had worse health in eight of twelve categories, including gum disease and lung function. Interestingly, marijuana use had better results in areas including cholesterol and body mass index. But the study is of a homogenous population in one country. Could negative effects show up after age 38, when the study stopped? Maybe. Plus, this test didn't measure cognitive health markers such as short term mental impairment.

After alcohol, marijuana is the drug most often found in the blood of drivers involved in accidents. As marijuana becomes more readily available legally, its effects need to be studied so we only pass laws based on sound scientific evidence and not emotions or fads.