• Vitamin A is highly effective in preventing a flu from taking hold. The recommended dosage is 100,000 IUs at onset and 100,000 IUs in the evening. Patients usually feel great the next morning. If more in-depth immune support is required 100,000 IUs can be taken twice a day for no longer than 1 week – after this liver toxicity can become a problem.
• Niacin flush is usually not a problem in dosages of 100 mg or less. It can be minimized by gradually increasing the dosage, taking it right after a meal, and accompanying it with vitamin-C. Talking aspirin for 2 days before beginning niacin therapy will also reduce flushing intensity. Once the first flush has been experienced aspirin is no longer required.
• Although niacin is not toxic to the liver it can result in elevated liver function tests. This effect can be avoided by taking 1.2 g of lecithin twice daily along with the niacin.
• Niacin may increase homocysteine levels, but this effect is easily counteracted by supplementation with folic acid, vitamin B6 and vitamin B12.
• Vitamin-B6 deficiency is common and associated with elevated homocysteine levels, premenstrual syndrome (PMS), kidney stones, and asthma. Supplementation with 40-100 mg/day of vitamin B6 (pyridoxine) helps alleviate the above conditions.
• Total body sun exposure provides the equivalent of 10,000 IUs (250 micrograms) of vitamin D. Thus the safe limit for daily vitamin-D intake is probably around 10,000 IU/day. Dr. Reinhold Veith, a noted vitamin D researcher, says that many people are deficient in vitamin D, particularly those living in northern latitudes. He believes the current recommended daily intake of 400 IU/day is too low by a factor of 10. Vitamin D deficiency has been associated with osteoporosis, rickets, multiple sclerosis, hypertension, congestive heart failure, several types of cancer, and scleroderma.
• Vitamin E helps normalize blood pressure, strengthens and regulates the heart beat, helps relieve angina, prolongs prothrombin clotting time, and decreases platelet adhesion. Normal dosage is 400-800 IU/day, but up to 3,000 IU/day may safely be used to treat specific conditions. A dose of 2,000 IU/day was used safely in elderly patients to slow down the progression of Alzheimer's disease.

The overall conclusion of the reports is that vitamins have an important role in both the prevention and treatment of many disorders, that they are generally safe, and that not a single fatality has ever been associated with vitamin supplementation.
The safety and efficacy of vitamins. Journal of Orthomolecular Medicine, Special Issue, Vol. 18, 3rd & 4th Quarters, 2003