A method of treating Alzheimer's disease, Parkinson's disease, sexual dysfunction or erectile dysfunction in a man by administration of a 5alpha reductase inhibitor together with a testosterone supplement is described. The method is also concerned with the use of the 5alpha reductase inhibiting compound and the testosterone supplement together with another agent useful for treating erectile dysfunction, including PDE V inhibitors; AGE (advanced glycation end-product) breakers; alpha 1 blockers; alpha 1A antagonists; alpha 2 antagonists; dopamine agonists; dopamine D4 agonists; melanocortin agonists; oxytocin agonists; prostaglandin; radical scavengers; rotamase inhibitors; aviptadil; nitroglycerine; and GPCR agonists for treating male sexual dysfunction or erectile dysfunction.