I now take OTC vitamin D3 with K2. D3 is recommended over D2. I have never taken a prescription dose. My vitamin D3 is 125 mcg which supplies 5000 IU of vitamin D. Before I began taking vitamin D on a daily basis, my labs always showed readings of only high 20s even though I spent lots of time outdoors in the Florida sun. At age 76, our bodies no longer absorb as well from the sun. My most recent vitamin D blood labs showed reading of 54. Amazing and a first for me. I also have gastric issues such as you, although mine is chronic pancreatitis with a lot of absorption issues, which no doubt contributed to my osteoporosis. But with my current vitamin D3, the numbers have gone well into the normal desirable range and my doctor has instructed me now to take my D3 every other day to maintain . If the numbers go too high, there can be a toxic effect. I always take D3 with food along with my enzyme that helps me to absorb my nutrients; without that enzyme. I would absorb very little of benefit. You might also benefit from monitoring the IBS and the foods that trigger symptoms. No doubt it is affecting your absorption. Another poster recommended magnesium, which I also take and I use magnesium citrate, which has maximum absorption benefits. This is quite a balancing act for all of us. I also take calcium citrate, and your body can only utilize just so much calcium at a time. I am careful to spread that over the day and not interact with my prescription meds. What a “nuisance”! With your IBS, the citrates Might trigger diarrhea so on the magnesium glycinate could be a better choice. Each of us has to try our own vitamin or prescription regime and see what works best. There are general guidelines, of course but as individuals we have to work it out.