We have previously shown that kava and its flavokavain-free Fraction B completely blocked 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenesis in A/J mice with a preferential reduction in NNK-induced O(6)-methylguanine (O(6)-mG). In this study, we first identified natural (+)-dihydromethysticin (DHM) as a lead compound through evaluating the in vivo efficacy of five major compounds in Fraction B on reducing O(6)-mG in lung tissues. (+)-DHM demonstrated outstanding chemopreventive activity against NNK-induced lung tumorigenesis in A/J mice with 97% reduction of adenoma multiplicity at a dose of 0.05 mg/g of diet (50 ppm). Synthetic (±)-DHM was equally effective as the natural (+)-DHM in these bioassays while a structurally similar analog, (+)-dihydrokavain (DHK), was completely inactive, revealing a sharp in vivo structure-activity relationship (SAR). Analyses of an expanded panel of NNK-induced DNA adducts revealed that DHM reduced a subset of DNA adducts in lung tissues derived from4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL, the active metabolite of NNK). Preliminary 17-week safety studies of DHM in A/J mice at a dose of 0.5 mg/g of diet (at least 10x its minimum effective dose) revealed no adverse effects, suggesting that DHM is likely free of kava's hepatotoxic risk. These results demonstrate the outstanding efficacy and promising safety margin of DHM in preventing NNK-induced lung tumorigenesis in A/J mice, with a unique mechanism of action and high target specificity.