An oral dietary supplement containing a natural cyclic monoterpene, and a major component of the oil extracted from citrus peels, with potential chemopreventive and antineoplastic activities. Upon oral administration, D-limonene activates aldehyde dehydrogenase 3A1 (ALDH3A1), thereby decreasing aldehyde level. This may protect salivary stem/progenitor cells (SSPCs) from toxic aldehydes and prevent or improve radiation-induced xerostomia. Limonene and its metabolites perillic acid, dihydroperillic acid, uroterpenol and limonene 1,2-diol may also inhibit tumor growth through inhibition of p21-dependent signaling and may induce apoptosis via the induction of the transforming growth factor beta-signaling pathway. In addition, they inhibit post-translational modification of signal transduction proteins, resulting in G1 cell cycle arrest as well as differential expression of cell cycle- and apoptosis-related genes.