OPRM1 and periodontitis: Periodontitis in the presence of OPRM1 genetic variation was less likely to occur in smokers in comparisons 2—Heavy smokers versus Light Smokers, 5—Heavy Smokers versus Light Smokers, Ex and Non-Smokers, and 6—Heavy Smokers versus Light Smokers and Ex-Smokers, suggesting that heavy smokers were less likely to have periodontitis, when compared to light smokers.