Dosage forms and methods of use are disclosed for: a) the simultaneous administration of tetrahydrobiopterin (BH4) or a derivative, homolog or precursor thereof and lipoic acid (LA), or dihydrolipoic acid (DHLA), or a derivative, homolog or salt thereof or, b) the administration of a conjugate consisting of tetrahydrobiopterin bis-lipoate (TBL). The invention is useful for the amelioration of diabetes mellitus types 1 and 2 (including impaired glucose tolerance, pre-diabetes, insulin resistance, metabolic syndrome X and as an adjunct to oral antidiabetic agents and/or insulin), diabetic and non-diabetic microvascular diseases (including nephropathy, neuropathy and retinopathy), diabetic and non-diabetic macrovascular diseases (including heart attack, stroke, peripheral vascular disease and ischemia-reperfusion injury), hypertension, vasoconstriction, obesity, dyslipedemia, and neurodegenerative disorders (including Parkinson's disease, mild cognitive impairment, senile dementia, Alzheimer's disease, hearing loss and chronic glaucomas). A novel method for the preparation of TBL is also disclosed.