Elevated levels of homocysteine have been implicated as an important risk factor for cardiovascular and other diseases. A composition for decreasing levels of plasma homocysteine and a method for administering the composition are provided the composition containing dextromethorphan (DM), folic acid and vitamins B6 and B12. The composition provides a synergistic therapeutic effect so that lower amounts of the above ingredients may be employed to minimize any undesirable side effects caused by the use of high levels of a component such as DM. Preferred compositions for cardiovascular diseases further include lecithin, vitamin E, beta-carotene, procyanidins/flavonoids, trimethylglycine, garlic oil and minerals. Other compositions for treating glaucoma include bilberry, bioflavonoids and beta-carotene and for treating tardive dyskinesia include an antioxidant such as grape seed extract and pine bark extract, lecithin and oligomeric proanthocyanidins. The compositions may be administered using any suitable means such as orally or intravenous.