Sorry if this is long, but I'm hoping I can help someone who was in my situation. Last fall I was diagnosed with a vitamin D deficiency (level 16). I had been dealing with horrible body aches for years (don't ask me why it took me so long to go to the doctor). Basically, I felt like I was 90 years old, even though I was in my early 30s. Everything hurt from my head to my toes. So, my doc prescribed 50,000 vit D2 per week for 12 weeks. Apparently this is the cookie cutter prescription/dosage for a vit D deficiency. 12 weeks later, I got my levels checked, and they'd gone down! I got home feeling super discouraged and started doing research. I found out that D3 is way more soluble than D2. So I started taking "Zhou K2 D3" (also on Amazon) and I felt better INSTANTLY. I take about 65,000 D3 per week. Suggested dosage charts can be found on the web. Within a few weeks I felt back to normal, which was AMAZING. Until.... About a month in, I started having HORRIBLE lower Back pain, which I'd never experienced before. I'm talking flaming hot knife into your back type of pain. I'm talking I literally had tears in my eyes in the Old Navy checkout line type of pain! Massage didn't help the pain either. It felt deeper inside. I started to think it was my kidneys, but it was a little too low on my back. So back to the web I went.... Turns out, you also need to take magnesium (and lots of it) if you're taking high dosage D3. Otherwise you can become deficient in magnesium, and what's the #1 symptom of magnesium deficiency? Flaming hot knife Back pain. I purchased "Doctor's Best High Absorption 100% Chelated Magnesium", and I take about 500mg daily. There are suggested dosage charts on the web for magnesium also. My Back pain went away the first day I started the magnesium. These Doctor's Best pills are on the large side, but they're not huge like some other reviewers have suggested. I have no problem taking them (and I even have thyroid nodules on my throat that make swallowing more difficult). I take them at night, because they make me sleepy. Now that I'm reading others' reviews, I'm thinking the anxiety I was experiencing (around the time of the Back pain) was probably related to the magnesium deficiency as well. I FINALLY feel cured, and I am sooo glad I've found the right dosage and combo of vitamins. Thank you Doctor's Best, thank you amazon reviewers and most of all, thank you to the interwebs