Dai (2018): Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial https://pubmed.ncbi.nlm.nih.gov/30541089/

"Our findings suggest that optimal magnesium status may be important for optimizing 25(OH)D status. "

OTC vitamin D is effective, recommended dose is 600IU per day: https://en.wikipedia.org/wiki/Vitamin_D#Recommended_levels

In my experience, it is certainly effective. I take 2000IU fairly regularly since last few years and it certainly helped to keep vitamin D level slight above minimum required.