The RX doesn't work. New studies show it 87% less effect than D3 which is OTC.

The general rule is 1000 IU D3 for each 10 points you need to raise. In your case 5,000 IU D3 daily would be a good start.
People who have more fat on them, sometimes need more.

The capsules for the OTC are tiny, and easy to tolerate, unlike the RX one which is D2. They typically cost about $5.00 a month... very little.

There is a condition called MTHFR mutation, that results in B12 and folic acid not being methylated properly in the tissues, and hence they will show up in tests floating around but not be the active form. So I would suggest you try methylB12 orally for 3 months to see if you detect any differences. If you do, then you will need both methylB12 and methylfolate for life. 10-30% have this error. It leads to elevated homocysteine which can cause STROKES and blood clots! Which seem to be your problem. You can get a MMA test and/or homocysteine test and if either or both are elevated, then your B12 is not working.