Vitamin D3 is the most bioavailable form of vitamin D, which is well known by both alternative and mainstream medical practitioners to have bone-protective properties. I use this particular brand because the company is highly reputable. There are varying opinions on how much D3 to take. As you can see, this product is 5000 IU. My own alternative healthcare provider has vacillated over the years between recommending a dose of 1000 IU per day, to 2000 IU, to the 5000 IU of this product. I personally take between 1000 - 5000 IU per day depending on the time of year. Regarding a potential danger of overdose of vitamin D, the body itself produces up to 20,000 IU of vitamin D after just 20 minutes of unprotected sun exposure in the summer, with no ill effects. However, for those embarking on a dosage of more than 1000 IU daily, ideally you would do it with a doctor's supervision, having the doctor regularly test your blood levels of vitamin D. Your doctor can test them by ordering a lab to do the 25-hydroxy vitamin D test, abbreviated as "25(OH)D." This is touted as the most reliable test of vitamin D status. Optimal levels range between 50 and 70 ng/mL Of course, if you are willing to allow a few minutes of sun exposure in the summer, you can get all the vitamin D you need that way during that time of year. However, in the winter, the angle of the sun is such that if you don't live quite near the equator, the body produces inadequate vitamin D when exposed to the sun--not to mention that very little of the skin is exposed to the sun when it is severely cold outside. Research has discovered that Americans as a whole are dangerously deficient in vitamin D, especially during winter months. The body's ability to produce vitamin D naturally also declines with age. In addition to its well-known benefit of protecting bones, vitamin D encourages the body to produce substances called cathelicidins which destroy viruses, bacteria, and other microbes, which is why vitamin D increases resistance to colds and flu. Research has shown that people who take cod liver oil lower their risk for respiratory infections of all kinds. (Apparently this 19th Century cure-all actually was a viable home remedy.) The risks associated with suboptimal blood levels of vitamin D are many, including: high blood pressure, increased cancer risk, metabolic syndrome, diabetes, and obesity. The benefits of supplementation and the risks of deficiencies of vitamin D increase as we age. In particular, in senior citizens vitamin D deficiency is linked with generalized bodily weakness, an increased possibility of developing macular degeneration, and a 2.5 times greater risk of death. In contrast, research has indicated that supplementing with vitamin D decreases the risk of developing cancer of the colon, lungs, and breast. It also improves balance in old people--which is a great thing since older people are at substantial risk of death from a fall. It also improves mood, and as such is a natural treatment for depression (it is well known that depression rates increase in winter due to lesser amounts of sunlight). Studies have shown vitamin D improves symptoms of all kinds of autoimmune disorders such as Psoriasis, and that it can relieve the pain of fibromyalgia. During flu season, vitamin D3 can help keep you from getting sick or lessen the duration of bouts of colds, sore throats or flu. My healthcare provider recommended to me taking as much as 10,000 IU on a day I begin to get a cold or flu. I've been on this supplement for years, along with 2000 IU of vitamin C daily, and haven't had a cold or flu since. Other than vitamin D3, other ingredients of this product include: rice flour, gelatin, sucrose, dialpha tocpherol, corn starch, and magnesium stearate. For those with allergies, there is a warning on the package: "Contains soybeans. Contains corn and rice." However, it certifies that, "This product contains NO milk, egg, fish, peanuts, crustacean shellfish, tree nuts, wheat, yeast or gluten. Contains NO artificial sweeteners, flavors, colors, or preservatives." This product comes with the following posted warnings: "Store tightly closed in a cool, dry place. Large doses (1400-2000 IU/day) may cause hypercalcemia, a decrease in renal function, and nephrocalcinosis. Monthly blood tests to monitor serum calcium and parathyroid hormone levels should be done to protect against vitamin D3 toxicity. Those monitor serum calcium and parathyroid hormone levels should be done to protect against vitamin D3 toxicity. Those with underlying kidney disease should avoid high doses of vitamin D3. Although some research indicates that dosages up to 10,000 IU/day are safe, healthy people should stay below 2000 IU/day. Keep out of reach of children. Do not exceed recommended dose. Do not purchase if outer seal is broken or damaged. When using nutritional supplements, please inform your physician if you are undergoing treatment for a medical condition or if you are pregnant or lactating.