My vitamin D levels were extremely low, so I purchased these to bring it back up. At the same time I had a horrible cold and cough. I took three of the D3 to start with, then I took one every three days as directed by my physician for a month. That brough my levels back up within that month and also helped get rid of my cold (took vitamin C as well for that). Now I take one pill every 5 days, again as directed by my physician. I had a bad winter with sickness, so every time I felt a cold/virus coming on, I took three with 4,000 units of Vitamin C for a couple of days, and that did stop the virus in its track. This is an excellent product. I highly recommend it. I followed my physician's directions EXACTLY (she is a Dr. Of functional medicine) as I know that she knows what she is doing. I now have other people in my family taking this after they spoke to their nutritionists about it. It's truly a great product that helped me build my immune system back up after some severe health issues. I am grateful for it.