I was put on prescription vitamin D2 50000 IU and it caused a bunch of side effects for me including heart palpitations for over a week and then a paradoxical reaction to magnesium causing them to be even more intense.

Proceed with caution and listen to your body. Doctors were accusing every other thing than accepting whatever it did to my calcium / other electrolytes bothered my heart.

It's expensive in the US because one company has exclusive sales here (patent protection?), but you could try calcefidiol, weekly dose and is supposed to get levels up rapidly. Apparently it's the common form to take in Spain, and it's further down the metabolic pathway vs cholecalciferol. (I take but still have to get levels checked)