Pharmaceutical compositions and methods of treatment or prevention of diseases and conditions associated with or characterized by at least one of xerostomia and periodontal disease by modulation of the nitric oxide synthase pathway are provided, as are animal models and drug screening methods. Such modulation may be achieved by the administration of tetrahydrobiopterin (BH4) or its prodrugs and/or salts. Treating and preventing xerostomia can have the downstream effect of also preventing periodontitis, dental caries, parotid gland enlargement, inflammation and fissuring of the lips (chelitis), inflammation or ulcers of the tongue and buccal mucosa, oral candidiasis, salivary gland infection (sialadenitis), halitosis, and cracking and fissuring of oral mucosa.